The constrictor muscles of the mantle are often subannular, interrupted on one or the other surface, decussating in some, connected in others. The following are a few examples of the disposition of these muscular bands. In S. eristata, around the tubular anterior ori fice, are 4.-5 sphincter muscles ; the first of which, as well as the last two, are continuous around the tube. The last two send lateral processes backwards, forming, with the suc ceeding band, the first on the trunk, and also annular, large rhomboidal meshes ; two other annular bands succeed and form large meshes by intercommunications on the upper and lower surfaces. From these last muscles two intercommunicating processes are sent off on each side to the crestlike prominence of the test. From the last band several branches run backwards to be distributed to the upper and lower lips of the posterior orifices.
In S. Tilesii the anterior orifice is provided with two lateral penniform muscles. The other muscles form six bands nearly parallel with each other, interrupted along the me dian line of the superior surface, and not reaching further than halfway down the sides of the body. The most anterior band is somewhat forked, and the most posterior gives some branches to the superior lip of the posterior orifice, and joins, at the angle of the lips, a radiating group of other fascicles, that are distributed to the inferior lip.
In S. scutigera the muscular bands are few. On the superior surface, two pairs of decus sating muscles are seen, and other smaller bands occur near the extremities. In S. cy lindrica there are eleven transverse bands, interrupted and separated by a considerable space on the inferior surface of the body ; the first six of which are parallel one to another, whilst the four following are bent towards each other on the median line of the body ; the last band, and some short lateral fascicles, are arranged close by the posterior ex tremity. In S. fusiformis there are seven transverse bands ; some of which are parallel with, and approaching others on the sides of the animal, and others are more or less obliquely arranged. In S. cordiformis there are six broad transverse muscles on each side of the animal, not meeting on either surface. Two narrow, looplike fascicles, one above and one below, arising from the last branchial bands, and two short, transverse, lateral slips, act as sphincters to the posterior orifice. A pair of narrow muscular fascicles are sent off from the first of the branchial muscles to the anterior orifice. In S. zonaria (the
aggregate form or "proles " of S. cordiformis) there are also six transverse bands interrupted only on one surface, and differing consider ably from those of S. cordiformis, as do the distinct sets of muscles distributed to the two orifices. In S.runcinata (solitary) (fig.772. the muscular bands are nine in number, placed on the dorsal surface ; three anterior and three posterior, approximated at their centres, and three parallel bands in the middle. In S. runcinata (aggregate) (fig. 772.B) there are six muscles, besides those of the orifices ; four anterior and two posterior, approximating on the median lines.
The branchia is single, in the form of a rihand-like tube, stretched, on a vertical plane, obliquely across the central or branchial cavity of the body, so that it is constantly bathed by the water traversing this cavity. It consists of a double membrane formed by a fold of the internal tunic or mantle, and springs anteriorly from the visceral nucleus between the oeso phageal opening and the orifice of the rectum ; it then becomes free, and ultimately terminates on the superior part of the thoracic cavity, below the point where the nerve-ganglion and the oculiform organ are found. It thus di vides the branchial cavity into two portions —the antero-dorsal or pharyngeal, and the postero-ventral or cloacal.
The inferior border of the branchia exhibits an infinite number of minute transverse ves sels, all parallel to each other. There is usually only a single row of these transverse vessels on each side of the foliole of the bran chia ; but sometimes there are many rows, which indicate the presence of many longi tudinal vessels, and thereby approach the character of the branchial tissue of the As cidians.
Savigny observed in S. octofora another small branchia near the anterior insertion of the large branchia.
The whole inner surface of the branchial sac of the Salpm seems, from its high degree of vascularity, to be subservient to the purposes of respiration. The vascular tissue of the bran chia, consisting of longitudinal and transverse vessels, is equivalent to the vertical bran chial network dividing transversely the respi ratory cavity of the Clavellince.
The single branchial lamina of Sal/me ap pears to constitute the transition from the Ascidia• to the Teredines; in the latter there are two elongated branchial laminae above the intestine, and within the tubular mantle, to which the water has access and egress by means of two tubes placed at the posterior extremity of the body.