540 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



but by the byssus-threads, which are secreted by a 

 gland at the hinder end of the body. After a time 

 the Olochidium, as the larva at this stage is called, 

 breaks away from the parent, and makes it way to 

 some of the fish that live in the same water. To the 

 gills or other part of these hosts it fixes itself by its 

 toothed shell, while the byssus gland becomes aborted, 

 as do also the sense organs with which the larva is 

 provided. Attached to and covered by the epidermis 

 of its host, the young mussel undergoes a series of 

 further changes and takes on the characters of the 

 adult. 



When the Molluscan larva is referable to the 

 trochosphere type, it has, as Lankester was the first 

 to point out, two distinctive characteristics ; on the 

 ventral surface, between the mouth and the anus, 

 there is a projection which is the rudiment of the 

 foot, and on the dorsal surface there is an epiblastic 

 ingrowth which forms the shell gland. The larva of 

 Chiton is remarkable for having the posterior dorsal 

 region segmented. 



The simplest of all known larvae are found in the 

 Coelenterata, where they have the form of a two- 

 layered oval or elongated body, covered externally 

 with cilia, and provided with a central gastric cavity, 

 but without a mouth. In the simplest cases this 

 Plan 11 1 a becomes fixed by one end, loses its cilia, and 

 begins to develop tentacles at its free end. In the 

 common jelly-fish (Aurelia) and in the vast majority 

 of the Acraspedota a very remarkable metamorphosis 

 obtains. The free-swimming planula having settled 

 down and become fixed (Scyphistoma stage) in 

 the form of a polyp with a central mouth (Fig. 225 ; A), 

 begins to undergo division into a number of saucer- 

 like rings set one below the other ; each of these 

 Strobila contains a portion of the gastric cavity, and, 

 as development proceeds, the edges of the saucers 



