102 THE VOYAGE OF H.M.S. CHALLENGER. 



tenuis) leaves the stomach on its posterior end, and curves ventrally and then anteriorly 

 to open to the exterior (Fig. 13, a) in the middle ventral line in front of the pair of 

 atrial apertures. This independent opening of the intestine upon the surface of the 

 body is just what might be expected from the incomplete and embryonic condition of 

 the peribranchial cavity. The heart is placed in the middle line of the Ijody ventrally 

 to the stomach and near the posterior end of the branchial cavity. According to 

 Lankester, it is formed of two cells which are i)laced at the opposite ends, and are 

 connected by delicate contractile protoplasmic fibrils. 



The nervous system consists of a large (cerebral) ganglion, which is placed near the 

 anterior end of the body on the dorsal edge of the branchial sac (see Fig. 13, n.g.). 

 This corresponds to the single ganglion of ordinary Ascidians. From this cerebral 

 ganglion a nerve cord runs backwards along the dorsal edge of the branchial sac, and 

 then curves ventrally and passes the alimentary viscera to reach the tail, along which 

 it runs on the left side of the urochord. Near the base of the tail there is a distinct 

 elongated caudal ganglion (Fig. 13, n.g.') on the nerve cord, and farther on there 

 are the slight enlargements already referred to. All these are connected with 

 distributory nerves, and Langerhans^ has described both motor and sensory nerves 

 in the tail. 



There are various sense organs in the body. The cerebral ganglion has connected with 

 it an otocyst, a pigment spot, and a tubular process opening into the branchial sac, and 

 representing the dorsal tubercle and associated parts of an ordinary Ascidian. On the 

 ventral edge of the branchial aperture are placed some groups of modified ectodermal 

 cells, which are possibly tactile organs (Fig. 13, s.o.), and there may be others of the 

 same nature on other parts of the body. 



The reproductive organs are placed at the posterior end of the body. They consist 

 of either single or paired large sac-like ovaries and testes, the latter communicating with 

 the dorsal surface of the body by means of a delicate duct, while the ova are only set 

 free by rupture of the body-wall. The testis arrives at maturity before the ovary, so 

 that although hermaphrodite, the Appendiculariida3 are not self-fertilizing. This is a 

 case of protanclry, while in most other Tunicata which I have examined- protogyny 

 appears to be the rule. 



Very little is known as yet in regard to the development. There is no reproduction 

 by gemmation, and therefore no alternation of generations in the life-histories of this 

 group. 



The family contains about two dozen species, which may be grouped in the 

 following four genera : — 



^ Zeitschr.f. ivi$$, ZuoL, Bd. xxxiv. p. 14-A. 



