TUNICATA 675 



and intestine as a compact mass ; the testis ramifies over the stomach 

 and intestine. The genital ducts run side by side along the rectum and 

 at some distance beyond its end open into the cloaca. The vas de- 

 ferens is the narrower, and has a patch of red excretory cells around 

 its enlarged end, where it opens by a rosette of small pores. Fertili- 

 zation takes place in the water, and the spermatozoa will not unite 

 with ova from the same individual. 



The central nervous system is reduced to a single elongated solid 

 ganglion (brain) on the dorsal side between mouth and atrial opening. 

 From its ends nerves are given off. There are no organs of special 

 sense unless the pigment spots between the lobes of the mouth be 

 functional for the perception of light. 



Not the least striking feature of the remarkable organization which 

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

 tainty 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 con- 

 cerning 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 differ 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 

 Appendicularia larva, and often as the "ascidian tadpole". This 

 creature (Fig. 472 A) has a tail about 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 swim.ming 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 phafynx, 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. 



