XVII 



HABITS OF LARVAE 



481 



arisen from its edge. In this stage the larva leaves its mother's 

 shell and swims out into the world of water to look for a suit- 

 able place on which to settle down. This is the only stage in the 

 life history of a Brachiopod when the animal is locomotor, and can 

 serve to spread its species. The extreme minuteness of the larva 

 and the short time it spends in this motile condition probably 

 accounts for the fact that Brachiopods are extremely localised. 

 Where they do occur they are found in great numbers, rocks 

 being often almost covered with them, but they are not found over 

 large areas. When viewed under a microscope the larvae seem 



Fig. 318.— Young larva of 

 Cistella neapolitana, 

 showing three seg- 

 ments, two eye-spots, 

 and two bundles of 

 setae. (After Kowa- 

 levsky.) 



Fig. 319. — Full-grown larva of Cis- 

 tella neapolitana, with umbrella- 

 shaped head, ciliated. (After 

 Kowalevsky.) 



to be moving with surprising rapidity, but judging from the 

 analogy of other forms, it seems doubtful if they swim a yard in 

 an hour. 



Frequently the larva stands on its head for some time, as if 

 investigating the nature of the rocks on which it may settle ; it 

 is extremely contractile, turning its head from time to time, 

 and seldom retaining the same outline for any length of time ; the 

 setae are protruded, and at times stick out in every direction ; 

 they are possibly defensive in function. When fully stretched 

 out the larva is about J mm. long, but it frequently shortens its 



VOL. Ill 



21 



