Ascidian

Ascidian, the Sea Squirt, is a good type of the class Virochorda, the lowest division of the great phyllum Chordata. The body is sac-like, and consists of two tunics perforated by a mouth and an "atrial pore."- The former leads to a large pharynx or branchial sac; this is lined by a network of longitudinal and transverse vessels. This network is respiratory in function, as water can pass through the pores (stigmata) between the vessels to the atrium; this is a cavity that nearly surrounds the pharynx, and it communicates to the exterior by the atrial pore. The alimentary system consists of an oesophagus leading from the pharynx to the stomach and intestine; the latter opens to the atrium. The single nerve ganglion is between the mouth and atrial pore, and beneath it a ciliated groove, the endostyle (q.v.), runs along the ventral edge of the pharynx; it is the relation of the nerve system and endostyle that gives the ascidian its vertebrate affinities. This is especially well shown in the embryo and such forms as Appendicularia.