394 INVERTEBEATA chap. 



gives rise to the sucker muscles, and are inserted in the cells of the 

 corona. 



During its active life the larva swims with its apical organ 

 directed forwards ; but when the free life draws to an end it glides 

 over the bottom, with its oral surface directed downwards, and during 

 this period the vibratile plume can be seen to carry out tactile move- 

 ments. Finally, the sucker is everted and forms a thin flat plate of 

 cells which adheres to the substratum (Fig. 315). All the muscles 

 except the sucker muscles contract strongly, and the piriform and 

 apical organs are in consequence strongly retracted. The outer edges 

 of the adhesive sucker turn upwards and unite with the edges of the 

 mantle, and the remnant of the atrial cavity is converted into a 

 ring-shaped space, towards the inner side of which the cilia are 

 directed. Then the muscles connecting the sucker with the valves 

 of the shell contract, and with great strength, so that the valves of 

 the shell are, so to speak, flattened out over the compressed larva. 



Histolysis of the larval tissues now begins. First the cushion 

 cells disintegrate and their mucoid contents are cast into the ring- 

 shaped atrium. In this way the ciliated cells of the corona become 

 cut loose from the mantle edge, the cells of which join the edge of 

 the adhesive plate formed from the sucker ; and the remnants of 

 the coronal cells are found floating in the ring-shaped atrial cavity. 

 The apical organ is very deeply invaginated and broken loose from 

 the flanking cushion cells; the adjacent ordinary ectoderm cells 

 meet above it, and from it, afterwards, the polypide of the first bud, 

 i.e. the alimentary canal and ciliated tentacles, are developed. The 

 coronal cells and larval muscles are attacked by wandering amoebo- 

 cytes. The stomach and intestines excrete brown granules into 

 their respective cavities, and finally lose their cavities and become 

 solid clumps of degenerating cells. The whole animal is thus reduced 

 to a thin-walled sac containing, invaginated into it at one point, a 

 thick-walled sac, which is the former apical disc and is the rudiment 

 of the future polypide of the mother bud of the colony. 



TYPES OF POLYZOAN LAEVAE 



Before studying the further development of the polypide it will 

 be well to cast a brief glance at the other types of larvae which have 

 been described in Polyzoa. All are modifications, one might add 

 modifications in the direction of degeneracy, of the Cyphonautes type. 



Prouho has indeed shown that the species Alcyonidium alhidum, 

 which belongs to quite a different division of Polyzoa (Ctenostomata) 

 from that to which Memhranipora belongs (Cheilostomata), has a larva 

 which can be distinguished only by minute specific differences from 

 the larva of Memhranipora ; and the same is true of the larva of 

 Hypophorella expansa, which also belongs to the Ctenostomata but to 

 a different division from that to which Alcyonidium belongs. 



The larva of Flustrella, a genus allied to Alcyonidium, is very 



