TUNICATA 597 



Not the least striking feature of the remarkable organization which 

 has just been described is the absence of any space that can with 

 certainty be identified as coelom. Epicardium, pericardium, the 

 cavities of the gonads, and even those of the closed excretory vesicles 

 that lie around the intestine in many ascidians have been held to be of 

 that nature, but there is no uncontrovertible evidence on this point 

 concerning any of them. Nephridia are also absent. Excretion, so far 

 as is known, is performed only by the cells mentioned above, which 

 store urates as solid concretions. 



So far, the student will have seen little ground for regarding Ciona 

 as a chordate animal. When, however, we turn to consider its life 

 history, no doubt remains upon this point. The eggs are small, though 

 they contain some yolk ; their cleavage is total and at first nearly equal. 

 The early stages of development much resemble those of Amphioxus, 

 but diifer in that the cells which are to form the rudiments of various 

 organs are very early recognizable (determinate cleavage), and that 

 the mesoderm, which arises from the sides of the archenteron, does so, 

 not as pouches, but as clumps of cells. Eventually there is formed 

 a larva, about a quarter of an inch in length, which is known as 

 the Appendicular ia larva, and often as the "ascidian tadpole". This 

 creature (Fig. 447 A) has a tail 2ihou.t four times as long as its trunk. In 

 the tail are a notochord, a hollow dorsal nerve cord, a muscle band on 

 each side, and a few mesenchyme cells. Dorsal and ventral median 

 flaps of the test serve as fins, the tail being a swimming organ. In the 

 trunk, notochord and muscle bands are lacking, and along with the 

 alimentary canal the brain and pericardium are found. The mouth lies 

 dorsally at some little distance from the front end. It leads through a 

 short oesophagus into a large pharynx, in which the endostyle is already 

 well developed. There is no branchial basket work, but on each side 

 a gill slit leads from the pharynx into an ectodermal pouch, which in 

 turn opens dorsolaterally. Later the two pouches become united 

 above the pharynx and thus the atrium comes into existence. Mean- 

 while the gill clefts increase in number by the breaking through from 

 the pharynx of new clefts and the subdivision of existing clefts, in the 

 course of which they pass through a U -stage with tongue bars. (The 

 basket work is ultimately established by the formation across each gill 

 cleft of longitudinal bars which divide it into stigmata.) From the 

 pharynx leads the rest of the alimentary canal, which early shows 

 rudiments of oesophagus, stomach, and intestine, the latter curving 

 dorsally and eventually opening into the left half of the atrium. 



The dorsal nerve tube of the tail is in the trunk enlarged to form 

 the brain. The hinder part of this is thick-walled and is known as the 

 trunk ganglion (it does not become the "ganglion" of the adult). The 

 anterior part is larger than the trunk ganglion and for the most part 



