66 



MOLLUSCA. 



Argonauta argo) the hectocotylised arm of the male appears as an 

 individualized intromittent organ which is filled with spermatophores, 

 then separates from the body of the male, moves about for a time 

 independently, and finally conveys the semen into the mantle cavity 

 of the female (fig. 539). 



The development* of the egg is introduced by a discoidal (partial) 

 segmentation which takes place at the pointed pole of the egg. As 

 in the bird's egg, the segmented portion of the ovum (formative yolk) 

 gives rise to a germinal disc which in the subsequent growth is 

 raised more and more from the lovei part of the blastoderm which 

 forms the yolk sac. Soon several projections appear on the embryonic 



rudiment (fig. 540) ; first in the 

 centre of the germ a flattened 

 ridge is formed around a cen- 

 tral depression (M) which it 

 soon grows over. This is the 

 mantle [the depression is the 

 so-called shell gland] ; on each 

 side of it the two parts of the 

 funnel appear (Tr\ and between 

 these and the mantle the gills 

 (Br). Also laterally but ex- 

 ternal to the folds of the funnel 

 the first traces of the head ap- 

 pear as two pairs of elongated 

 lobes, of which the external an- 

 terior pair bears the eyes. On 



FIG. 539.- Male of Argonauta argo (after H. the Outer edge of the disc papilli- 



Miiiier). He, hectocotylised arm. f orm structures are formed, the 



first rudiments of the arms. In the later growth of this absolutely 

 symmetrical embryo the Cephalopod form becomes more and more 

 apparent : the mantle projects considerably, and grows over the 

 gills and two parts of the funnel, which fuse to form the definitive 

 funnel. The cephalic lobes grow together between the mouth and 

 funnel, and on their oral sides become more sharply constricted off 

 from the yolk, which with a few exceptions persists for some time 

 as a yolk sac (fig. 541). 



The Cephalopods are marine animals, some frequenting the coast and 

 others the high seas. They feed on the flesh of other animals, 



* Cf . besides van Beneden and Kblliker ; Ussow, " Zoologisch-embryologische 

 Untersuchungen." ArcJiivfiir Naturgescli., 1874. 



