Tunicata

The reproductive organs, as in all Enterogona, consist of a single lobulate ovary and testis, each provided with its own duct, which runs alongside the intestine to open into the cloaca. The two glands occupy the typical position in the loop of the intestine. Among the Pleurogonous Ascidians the loop of the intestine tends to open out, and the lobules of the gonads become independent and dispersed (polycarps) over the wall of the atrial cavities, in some cases with a remarkable degree of metameric regularity, recalling that in Amphioxus.

The Endopharyngeal Apparatus.

The endostyle has a characteristic and very constant structure throughout the Asci diacea. It is a deep groove with high upstanding edges, which are strongly ciliated as the marginal bands, the general pharyngeal epithelium being unciliated. The bands unite behind the endostyle, but at the anterior end they diverge right and left and encircle the pharynx in front of the first row of stigmata as the peri pharyngeal bands. In the mid-dorsal line they unite once more and form a single ciliated tract which runs backwards as the or hyper-pharyngeal band until it merges in the general cilia tion at the entrance to the oesophagus. On one side the dorsal edges of the various horizontal membranes are produced as a series of dorsal languets, which in life are recurved upon them selves like so many flexible hooks. The edges of the horizontal membranes are ciliated. Between the endostyle and the oesopha geal aperture runs the retropharyngeal band, also ciliated.

Within the groove of the endostyle run 3 pairs of longitudinal tracts of gland-cells, of which the two lower pairs resemble one another in their wedge-like section, and may be distinguished as the primary tracts, comparable with those in Amphioxus, while the upper pair, consisting of cylindrical cells, constitute the ac cessory tracts. When any reduction takes place, these are the first to go (e.g., Distaplia, Doliolum, lasince). The three tracts are separated from one another on each side by two narrow bands of minute ciliated cells (the intermediate bands). The floor of the groove is composed of a few rows of small cells bearing long flagella, which reach to the top of the groove. These dis appear only in the Copelata.

This apparatus works as follows. The gland cells pour into the groove a constant stream of mucus-like slime, and, apparently by the action of the long median flagella, this is thrown out into the general cavity of the pharynx. The cilia bordering the stigmata

beat outwards into the atrial cavities, and cause a strong stream of water to pour in through the open mouth. The slime from the endostyle is carried with the water towards the nearest gill-slits (i.e., the ventral ones), but is at once worked into the form of longitudinal threads which, alighting upon the ciliated edges of the horizontal membranes, are swept dorsally, and entangle suspended food-particles as they are rolled upwards in rapid succession. The combined marginal and peripharyngeal bands convey a separate stream of mucus directly round the front end of the pharynx into the hyper-pharyngeal groove, along which it streams backwards to the oesophageal aperture, supported by the dorsal languets. Fringes of slime trailed off the peripharyngeal bands connect with the series rolled up from the endostyle, and all become entwined together dorsally into a single rotating cord of mingled slime and food-particles which passes uninterruptedly into the oesophagus.

In Clavelina, owing to its perfect transparency, this process can be readily observed by adding powdered carmine to the sea water round a living example under a binocular microscope. In other types of Ascidians the phenomena cannot be followed so completely, but there is good reason to believe that all the elaborations of the Ascidian pharynx are so many contrivances to the same end. The longitudinal bars and terminal papillae of Stolidobranchs and Phlebobranchs assist the process by muscular means, swinging to and fro transversely in such a way as mechan ically to pick up the longitudinal slime-ropes and push them upwards from one bar or papilla to the next. The concave "minute plications" or "meshes" of Phlebobranchs, and the longitudinal folds of Stolidobranchs subserve the same purpose by withdrawing the perforated surface of the pharynx away from the collecting surface, and by concentrating the longitudinal bars of the latter into closer proximity. In this way a secondary col lecting sieve is produced within the primary current-producing one—which enables a stronger stream to be poured through the pharynx without waste of food-particles. The formation of this inner cyclinder of muscular bars and ciliated ridges explains the elevation of the side-walls of the endostyle to its own level.