60 



LOWER VERTEBRATES. 



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

 two kinds differing considerably in appearance 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 amphij)od crusta- 

 ceans, Hyperia by name, uses this dead 

 ¥i(i. 61.— Doliolum. rieiiticuiaium: a, circular mus- test as a home, liviiiEf in it much like a 



cles of pharynx; v, eialostyle; ff, ganglion; A, , . , . . ^_. 



heart; i, intestine; m, mouth; o, oesopliagus; hermit Crab III ItS shell. One SpeCieS of 



r, reproductive organ; 5, stomach; a;, branchiae. i , , 



the genus has been seen on our shores, but 

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



Oedek 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 hoops. 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 cliaracter, the pro- 

 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 repro- 

 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 compar- 

 able to the placenta of the higher vertebrates. 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 Ave started. Thus the process 

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

 German poet-naturalist, Chamisso, who first discovered this 

 extraordinary mode of rejaroduction, "a Salpa-xaolher is not 

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

 its grandmother." 



Usually it is thought that the solitary Salpaj are asexual and that the chains are 

 sexual ; but Dr. W. K. Brooks, who has studied their development -s-ery profoundly, 

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

 places an egg in each of the chain salpae, 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. 52. — Salpa spinnsa, one in- 

 dividual from a chain: a, anus; 

 6, gill; r/, ganglion; h, heart; 

 tn, mouth; ii, nucleus; p, pro- 

 cesses by which the various 

 individuals of the chain are 

 connected together. 



