Fig. 108. Embryo of Cavolinia (Hyalea) Tridentata. (After Fol.)
m. mouth; a. anus; s. stomach; i. intestine; σ. nutritive sack; mb. mantle; mc. mantle cavity; Kn. contractile sinus; h. heart; r. renal sack: f. foot; pn. epipodia; q. shell; ot. otolithic sack.
Pteropoda. The early larval form of the Pteropods is closely similar to that of marine Gasteropods. There are usually only three hypoblastic spheres at the close of the segmentation in the Thecosomata, and a somewhat larger number in the Gymnosomata. The blastopore closes at the oral region, on the nutritive side of the ovum, and the shell-gland is placed at the original formative pole. The velum, shell-gland and foot have the usual relations. Although many of the adult forms are symmetrical, there is very early an asymmetry visible in the larva, shewing that the Pteropods are descended from asymmetrical ancestors. In the Gymnosomata there is a second larval stage after the loss of the shell when the larva is provided with three rings of cilia ([fig. 109]). In most forms of Pteropods the dorsal part of the body, covered by the mantle, is produced into a visceral sack like that of the Cephalopoda ([fig. 108]).
The velum varies considerably in its development in different forms. In the Hyaleidæ it is comparatively small and atrophies early; while in Cymbulia ([fig. 103]) and the Gymnosomata it is large and bilobed, and persists till after the foot has attained its full development.
The free edge of the velum is provided with long motor cilia, and its lower border with small cilia which bring the food to the mouth. In Cleodora there is a median bunch of cilia in the centre of the velum like that in the Lamellibranchiata, Nudibranchiata, etc.
The shell-gland forms a pit at the aboral end of the body, and in Cymbulia a chitinous plug appears to be normally formed in this pit. The pit afterwards everts itself. The edge of the everted area becomes thickened and gradually travels towards the anterior end of the body. On this everted area a small plate is developed, which forms the commencement of the embryonic shell with which the larvæ of all Pteropods are provided.
The remainder of the embryonic shell is secreted in successive rings by the thickened edge of the mantle, and grows with this till it reaches the neck ([fig. 108]). The permanent shell is added subsequently, usually on a very different model to the larval shell. The fate of the embryonic shell is very various in different forms. In the Hyaleidæ the animal withdraws itself from the larval shell, which becomes shut off from the permanent shell by a diaphragm. The larval shell then becomes detached.
Fig. 109. Free-swimming Pneumodermon Larvæ. (After Gegenbaur, copied from Bronn.)