440 AfisceUaneous. 



iiboral axis of the larva. The ectoderm closes up in the last place 

 at the point of union of the three spheres, a point which may coin- 

 eido either with the aboral or witli the oral pole of the larva. I 

 am in favour of tlu- latter alternative. 



The embryonic development of the Gyranosomes forms a transition 

 between that of the Thecosomes, which I have just recapitulated, 

 and that of the Hcteropoda, between tlie formation of the embryonic 

 lamellic by euvelo])meut and their formation ])y invaj^ination. 



The digestive cavity is formed by a simple differentiation of the 

 mass of nutritive or central cells. From this results a completely 

 closed trilobate cavity. From the median lobe i)roceeds the diges- 

 tive tube ; from the lateral lobes the nutritive sacs. The cells com- 

 posing the walls of this cavity descend directly from the nutritive or 

 central cells of the embryo ; they are small and immerous round the 

 median cavity ; cuneiform, and comjjosed in great part of imtritive 

 substance round the latend cavities. The median portion lengthens 

 to form the stomach and the intestine. An invagination uf the ecto- 

 derm, starting from the point where this lamella has closed up, de- 

 scends to meet the stomach, with which it unites. This invagination 

 represents the mouth and oesophagus ; the point of junction the 

 cardia. It represents in front a diverticulum which gives origin to 

 the radula. This development of tlie digestive tube agrees point by 

 point with what we know of the development of the llotifera. 



The first cilia which appear are motory ; they are in small tufts 

 on a circular zone on a level with the mouth ; then a band of small 

 cilia grows below the larger ones and serves to convey the nutritive 

 particles to the mouth. 



The foot has its origin in a thickening of the ectoderm, which 

 occupies the greater part of the ventral surface of the embryo. It 

 afterwards takes the form of a humj), and then that of a horizontal 

 tongue, which sometimes bears an operculum on its lower side. It 

 divides into a median lobe and two lateral ones, which become the 

 swimming-organs. 



The pallial cavity is formed by sinking-in of the ectoderm between 

 the edge of the shell and the neck of the larva, always on the right 

 of the anus whatever may be the position of the latter. 



The larva) of the Pteropoda have two contractile sinuses, situated 

 the one at the foot and the other in the dorsal region, which send 

 from one to the other the liquid contained in the cavity of the body. 

 Neither of these sinuses can be compared to those of the embryo of 

 the Limaces^. The cephalic sinus of the Limaces corresponds to all 

 ttie median portion of the velum and to the whole dorsal region of 

 the embrj'os of the Pteropoda. The conti-actilo sinus of the foot of 

 the Limaces is situated at the extremity, and not at the base of the 

 foot as in the Pteropoda. 



The kidney is formed at the expense of the ectoderm, and the 

 heart by the differentiation of a mass of cells of the mesoderm. 

 The internal aperture of the renal canal opens outside the heart and 

 into the pericardium when the latter is afterwards formed. The 

 kidney beats with almost as much vivacity as the heart. The aorta 

 and the arteries are formed by the differentiation of chains of meso- 

 dermic cells. 



