572 



SEMIVER TEBRA TES. 



Tailed Ascidians. 



ment. Usually, the family is divided into the two genera Salpa and Cyclosalpa, the 

 latter being distinguished by having the digestive tract coiled up ; but some writers 

 have divided the first of these two into several subgeneric groups. A second family 

 is represented by the very imperfectly known genus Octacnenius, dredged at depths 

 of between one and two thousand fathoms in the South Pacific ; the body being 

 much flattened, and probably attached by one extremity. Nothing is known as 

 to the life-history of this singular form. 



The second suborder — Cyclomyaria — of the free-swimming non-luminous as- 

 cidians takes its name from the muscular bands of the inner tunic forming perfect 

 rings, and is typically represented by the genus Doliolum. The life-history is 

 complicated by polymorphism ; the tailed larva developing into a sexless form, 

 the buds from which give rise to nutritive units, fostering units, and reproductive 

 units. In the typical genus all the muscles form encircling hoops, and the three 

 forms of the sexual generation occur together on one stolon, or outgrowth ; but in 

 Anchinia there are only two complete muscular rings, and the three forms of the 

 sexual generation are produced successively. 



The free-swimming form known as Appendicularia is the type 

 of the third and last order — Larvacea — of the class, all the members 

 of which are characterised by the possession in the adult state of large tail-like 

 appendages, furnished with a skeletal axis. These creatures, which are of minute 

 size, have not undergone the degeneration so noticeable in the adult of the other 

 tunicates, and thus correspond much more closely to the larval stage of the latter. 

 A curious feature is the rapid production of a temporary outer tunic, which may 

 be shed at any time, and replaced by a second one. There is. no separate atrial 

 cavity ; and the branchial chamber is simply an elongated pharynx, with two 

 openings on the lower surface, which correspond to the gill-slits, and are well 

 furnished with cilia. The nervous system consists of a large ganglion placed in 

 the anterior part of the dorsal surface, followed by a long chord, provided 

 with smaller ganglia, and extending backwards over the intestine to reach the 

 tail, where it runs along the left side of the skeletal axis. The intestine itself is 

 situated behind the branchial chamber, and the vent opens on the inferior or 

 ventral aspect of the body in advance of the gill-slits. Neither budding, meta- 

 morphosis, nor alternation of generations takes place ; and the reproductive organs 

 are situated at the hinder end of the body. The group comprises only the single 

 family Appendiculariidce, which contains five genera, the names and characters 

 of which it will be unnecessary to mention. 



Botryllus (nat. size and enlarged). 



