The velum has atrophied in both larvæ.
In A three ciliated bands are present, and the auditory vesicles are visible.
In B the tentacles with suckers and the epipodia have become developed.
an. anus.
In the Styliolidæ the permanent shell becomes twice the size of the embryonic shell while the animal is still in an embryonic condition, but the larval shell persists for life. In the Cymbulidæ there is an embryonic and secondary shell, which persist together during larval life. They are eventually cast off at the same time and replaced by a permanent shell.
In the Gymnosomata an embryonic shell is developed, and a secondary shell added to it during embryonic life. Both are cast off before the adult condition is attained. After the shell has been cast off three ciliated rings are developed ([fig. 109]). The anterior of these is placed between the velum and the foot, and the two hinder ones on the elongated posterior part of the body.
The ciliated rings give to these larvæ a resemblance to Chætopod larvæ; but there can be no doubt that this resemblance is a purely superficial one. The anterior ring atrophies early ([fig. 109] B), and the second one soon follows suit. It is probable that the hindermost one does not persist through life, although it has been observed in forms with fully developed sexual organs. Most of these larvæ have not been traced to their adult forms. They have been referred to Pneumodermon, Clio, etc.
The most characteristic organ of the Pteropods is the foot, which is prolonged into two enormous lateral wings, the epipodia. These develop at different periods in different larvæ, but are always distinct lateral outgrowths of the foot.
In the Hyaleidæ the foot is early conspicuous, and soon sends out two lateral prolongations ([fig. 108] pn.) which develop with enormous rapidity as compared with the medium portion, and give rise to the epipodia. The whole of the foot becomes ciliated.
In the Cymbulidæ, though not in other forms, an operculum is developed on the hinder surface of the foot ([fig. 103] C). The epipodia are late in appearing.
In the Gymnosomata the foot is developed very early, but remains small. The epipodia do not appear till very late in larval life ([fig. 109] B).
In Pneumodermon and some other Gymnosomata there appear on the hinder part of the head peculiar tentacles with suckers like those of the Cephalopoda ([fig. 109] B). It is not certain that these tentacles are genetically related to the arms of the Cephalopoda.