Part, second. 



Opposed to the sessile Ascidians we have the free-swimming Salpw. 

 The transparency of their bodies stamps them as pelagic animals, which 

 drift about like the jelly-fish on the open sea , and are occasionally 

 carried by currents to the coasts, where they very often find their way 

 into the nets of the fishermen , much against the wish of the latter. In 

 the Aquarium they will be almost always seen in spring and autumn 

 (tank Nr. 20) where the structure of the larger kinds , such as Salpa 

 maxima-africana (Fig. 172), may be readily followed. The outer wall 



Fig. 172. Single individual of Salpa maxima-africana, */2 nat. size. 



of the barrel-shaped body is like the tunic of the Ascidians , and pre- 

 sents a hole at each end. The throat is in Salpa , however , literally 

 nothing but gill-slits, its wall being represented only by a slanting bar 

 (observe vibrating hairs) leading from the mouth to the stomach across the 

 great swimming-cavity. When this is filled with water , the mouth is 

 closed, and the muscular bands , which surround the body like hoops 

 of a barrel, are forcibly contracted. This contraction forces the water 

 out through the hinder or outgoing pore and the animal is jerked 

 forwards. It thus sucks its way through the water. At the hinder end 

 of the body (left in Fig. 172) a brown globular mass , the stomach and 

 intestine , will be noticed. In front of it lies the transparent heart, 

 which in all Tunicata contracts for a few minutes from front to back and 

 then from back to front, so that the circulation of the blood is periodically 

 changed. 



The development of the Salpse is of great interest. The poet Cha- 

 misso, who was also a very good zoologist , was the first to observe on 

 his voyage round the world, that in Salpa the offspring does not resemble 

 the parent form, but the grand-parent. Thus in one species we have 

 two forms which alternate with each other (of. p. 61) ; one form he 

 found always as a single individual , whereas the other one occurred in 

 chains consisting of a large number of individaals. Later observations 

 have entirely confirmed the accounts of this Alternation of generations 

 of the Salpae. In the Aquarium visitors will find both chains (Fig. 94) 

 and single individuals (Fig. 93) of the same species ; the chains are 

 sometimes of considerable length , or they may form a closed ring 

 (Fig. 95). The members of such a chain are completely alike , and are 

 hermaphrodite. Their eggs never hatch into chains , but always into 

 single individuals, which are not only different anatomically from their 

 parent , bat also never produce eggs. Instead of these latter the ovary 



