320 



INVERTEBRATE MORPHOLOGY. 



the occurrence in the majoritj of forms of a larva known as 

 the Veliger (Fig. 144) which presents many interesting affin- 

 ities to the Annelid Trochophore. In the early stages of de- 

 velopment the embryo is strictly 

 bilateral, with the mouth and 

 anus at the extremities of the 

 longitudinal axis. Upon the dor- 

 sal surface posteriorly is a de- 

 pression lined with columnar cells 

 which secrete the larval shell (Sh), 

 and in front of this is an area 

 enclosed by two rows of cells 

 bearing stout cilia and forming 

 the velum {V). This band of 

 cilia is prssoral (Fro) in position, 

 and in addition to it a second 

 band of smaller cilia is to be 

 found which passes ventrally to 

 the mouth and constitutes a post- 

 oral band {Poo), the groove be- 

 tween it and the prseoral band 

 being occupied by the adoral cilia. On the ventral surface 

 is found a prominence which represents the foot. 



In later stages the lateral edges of the velum are drawn 

 out so as to form a broad lobe, sometimes divided into two 

 arms, projecting on each side of the head ; the prseoral and 

 postoral bauds of cilia extending round the margin of the 

 fold, not, however, completely enclosing the velar area, but re- 

 maining open on the dorsal surface. The shell area increases 

 markedly in size, the shell becoming spirally coiled, the vis- 

 ceral hump which develops in the shell area likewise assum- 

 ing the coiled form. At the margins of the shell area a fold 

 appears, the rudiment of the mantle, which gradually increases 

 in size as the shell area extends, and at the same time the 

 anus becomes rotated forwards from its original terminal posi- 

 tion along the right side of the body to a greater or less ex- 

 tent. As these changes progress, the embryo gradually ap- 

 proaches more and more to the adult form, differing from it 

 mainly in the existence of the velum, by means of which it 



Fig. 144.— Vbliger Larva. 

 F = foot. 

 M = mouth. 

 Oc = eye. 

 Poo = postoral band of cilia. 

 Pro = preeoral band of cilia. 

 Sh = shell. 

 2' = tentacle. 

 V = velum. 



