60 



LO WEH I 'Elt TEIillA TES. 



finally separate from the parent and float freely in the watei-. Of these buds there are 

 two kinds differing considerably in appeai'auce and origin. One kind develops into 



the sexual forms with which we began ; the 

 fate of the other is unknown. After a 

 longer or shorter time the individuals of 

 Doliolum die, and most of the soft parts 

 soon decay ; but the outer gelatinous walls 

 last for some time and fulfil a further use- 

 ful role; for one of the amphipod crusta- 

 ceans, Hyperia by name, uses this dead 

 test as a home, living in it much like a 

 hermit crab in its shell. One species of 

 the genus has been seen on our shores, but 

 it has not yet been described and honored with a Latin name. 



/, circulai inus- 

 u.M V lull/, g.ingliou. A, 



lieart; *, inti-htiiic, vi, uiuutli, o, asophrtgus, 

 r, reproductive organ, s, stomach, a:, braucui£e. 



Order II. — DESMOMYARIA. 



The members of this, the last order of the tunicates, diverges the most widely from 

 the type described in the foregoing pages. The body is nearly cylindrical, sometimes 

 flattened above and below. The muscular bands do not form complete hoojis. The 

 generation in these forms is still more complicated than in Doliolum, and authorities 

 are still in dispute concerning it. Leaving aside all mooted 

 questions, which are mostly of a technical character, the jiro- 

 cess seems to be much as follows : Two kind of individ- 

 uals occur, the solitary and the 'chain zooids,' the latter 

 being united together into long bands, the individuals of 

 which correspond to the links of the chain. From one of 

 the solitary individuals a long process or cord grows out, 

 and then this divides up to form the chain. Each one of the 

 chain zooids contains a single egg, and also the male re]iro- 

 ductive organs. The egg ripens before the spermatozoa, 

 becomes impregnated, and undergoes its development inside 

 the parent, to which it is connected by a structure com]iar- 

 able to the placenta of the higher veitebrates. After this 

 young Salpa is far along in its developmental history, the 

 male organs ripen and the spermatozoa are cast into the sea 

 to fertilize other eggs. Each egg finally forms a solitary 

 Salpa, like the one with which we started. Thus the jjrocess 

 is an alternation of generations, and, as expressed by the 

 German poet-naturalist, Chamisso, who first discovered this 

 extraordinary mode of reproduction, "a iSalpa-moiher is not 

 like its da\ighter, or its own mother, but resembles its sister, its grand-daughter, and 

 its grandmother." 



Usually it is thought that the solitary Sal|)ie are asexu.al and that the chains are 

 sexual ; but Dr. W. K. Brooks, who has studied their development very profoundly, 

 thinks otherwise. He regards the solitary individual as the female ; and says that it 

 places an egg in each of the chain salpse, while each member of the chain is to be re- 

 garded as a male. From this point of view no alternation of generation can occur. 



Fig. 62.— Salpa spinosa, one in- 

 dividual from a cliaiu : rt.anus; 

 fr, gill; //, ganglion; Ji, heart; 

 7», mouth; n, nucleus; p, pro- 

 cesses by wliich the various 

 individuals of the chain are 

 connected togetiier. 



