42 



EMBRYOLOGY 



made for the process of attachment.^ The larvae sink to the 

 bottom, their motions become slower, and finally they attach 

 themselves by means of the disc-like enlargement . of the 

 anterior end of the body (Fig. 15 G). They then lose their 

 cilia ; and the surface becomes covered with a thin cuticular 

 secretion, the perisarc (Fig. 15 D). The disc-shaped region 

 of attachment often acquires a lobed appearance, due to the 

 formation of notches (Fig. 15 E). The disc constitutes the 

 first fundament of the hydrorhiza, while the posterior end of 



Fig. 15. — Attachment and gi'owth of the hxrval planula of Endendrium (after 

 Allman). a, planula ; B and C, stages of attachment by means of the disc-like 

 enlargement of the anterior end ; D, beginning of the formation of the hydranth 

 and perisarc, p ; E, formation of the tentacles ; F, expansion of the hydranth, 



the larva, now directed upward, grows up into the first 

 hydranth of the young polyp colony. The fundaments of the 

 tentacles (Fig. 15 E) arise as lateral diverticula, and the 

 mouth-opening is formed at the apex by a breaking through 



' The development of the eggs of medusfe and of hydroid polyps with 

 gonophores is from the planula-stage onward alike, so that Eudendrium 

 may serve in this place as an example. 



