158 PROCEEDINGS OF THE ACADEMY OF [1897. 



In the fifth stage the larva appears divided into three regions : 

 anterior, middle and posterior. The anterior represents the cepha- 

 lic region, and is separated from the middle by a ring of cilia. The 

 collar covers the whole ventral side of the middle body region ; its 

 entire surface is ciliated. The body is of an orange color, while the 

 yolk mass is brown. There is a bright red projection below the 

 collar in the ventral region. Two ocelli are visible. 



The sixth larval stage is characterized by the greater develop- 

 ment of the collar and the development of hook-like setse in the 

 lateral portions of the body ; three pair of these seta? are arranged 

 between the collar and the posterior portion of the larval body. The 

 primitive body cavity lengthens during this stage. The region of 

 body from the posterior portion to mouth, becomes flattened. 



The body of the seventh larval stage is more vermiform ; the mid- 

 dle body region being the largest. Four ocelli are present (the 

 larger being the original two) which are placed on the apex of the 

 prostomium in the median line. There is an apical tuft of cilia 

 present and a ciliated post-oral ring. The mouth opens in the 

 median, ventral line and has ciliated lips. The collar is ciliated 

 and now covers only about one-third of the middle body region. 

 The posterior portion of the body is narrower than the middle por- 

 tion and is segmented ; its surface is ciliated on the ventral side 

 and there is a tuft of cilia on the last segment. The posterior end 

 of the intestine opens on the dorsal surface of the last segment by 

 means of an invagination of the ectoderm, and thus the anus is 

 formed. The operculum develops during this stage, appearing as a 

 narrow, triangular plate arising from the dorsal side of the head. 

 The larva now releases itself from the egg capsule by pressing its 

 spines against the walls, and becomes a free swimming form. 



In the eighth stage of the larva the operculum increases in size ; 

 the ocelli which were first to appear, now disappear ; the apical 

 ocelli remain. The club-shaped tentacles are replaced by the be- 

 ginning of branchiae. The operculum continues to develop. The 

 collar much reduced in size remains attached to the cephalic region. 

 In the ventral portion of the same region is the shell gland repre- 

 sented by an oblong mass of cells. The larva ceases to swim about 

 and sinks below the surface of the water, attaching itself to some 

 object by means of a translucent limy secretion. This is the shell 

 forming stage. 



