THE SEA SQUIRTS OR ASCIDIANS 2949 



latitude of Cape Horn and the most southern point of Australia. Dr. Brooks writes 

 that ' ' they are abundant only after the water has been for some time undisturbed 

 by winds; and as prolonged calms are most frequent in warm seas, those waters are 

 most favorable for the developement of these animals, which multiply with most 

 astonishing rapidity. The smaller species are often so abundant that for hundreds 

 of miles any bucketful of water dipped up at random, will be found to contain 

 hundreds of them. In such places collecting with the surface net becomes imprac- 

 ticable, for almost as soon as the net is dropped into the water, it becomes chocked 

 with a mass so dense that nothing can enter it. ' ' The food of these creatures con- 

 sists of minute marine organisms, both animal and vegetable. In swimming, chain 

 salpae progress by an undulating, snake-like movement. Usually, the family is 

 divided into the two genera Salpa and Cydosalpa, 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 Octacnemus, 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 nonluminous ascid- 

 ians takes its name from the muscular bands of the inner tunic forming perfect rings, 



Botryllus. 

 (Natural size and enlarged.) 



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 characterized 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; 



