MOLLUSCS. 



367 



capsules are united in a bunch, but I have seen one mass of the eggs of this species 

 which would more than fill a bushel basket. Owing to the great amount of food-yolk 

 present, the eggs of all cephalopods are very large, and the development is extensively 

 modified from the same reason. 



The development of but few of the cephalopods has been studied, and all that is 

 known throws but little light on the relationships of the group. A knowledge of the 

 development of Nautilus is a great desideratum. The follow- 

 ing account relates largely to Loligo pealei, the only species 

 studied by the writer. Owing to the great quantity of food 

 yolk and its distribution, the segmentation of the egg is at first 

 confined to one pole, and the result is that the first stages of the 

 blastoderm are very like those in the chick. The blastoderm 

 gradually increases until it envelojjes the whole yolk, but before 

 that stage is reached some of the organs begin to be outlined. 

 First to appear are a shallow pit, the shell gland, at the extrem- 

 ity of the body, and two others closely similar, the rudiments 

 of the eyes, on the sides of the body. Then a fold arises, the 

 first traces of the mantle. The two siphonal folds next appear, 

 and as develoj)ment pi'ogresses they take the form of two dis- 

 tinct flaps, a condition which is pei'manent in Nautilus, and 

 then the edges fuse togethei-. The first appearance of the arms 

 is in the shape of simple prominences. 

 At this time the yolk extends as a huge 

 mass from between the arms, but, as 

 gi'owth continues, it is gradually absorbed and transformed 

 into food for the rapidly increasing tissues. Thus the devel- 

 opment is direct, and not a trace of a metamorphosis appears. 

 Even the veliger and trochosphere conditions are lost. The 

 reasons for this are to be found in the very long history of the 

 group, together with the high point to which they have attained. 

 In their structure they are, as we have seen, the highest of the 

 Mollusca, and hence the transition from the egg to the adult is 

 very extensive. On this account there is a tendency. to drop 

 all useless and larval features, and to pursue the shortest 

 course. This tendency exists everywhere in the animal king- fio. 4S4. — Embryo squid in 

 dom, but of course it requires time to eliminate them all. have miitedfami'the sucic- 

 Time the cephalopods have had. In the lower Silurian rocks, peadngupoft™ pairs"of 

 fossil forms not generically distinguishable from living Nati- ^fin^^'^^a"; ^«'Tent.' ^^^' 

 till are found, and the tetrabranchs flourished in palaeozoic 

 time, while forms assigned to the dibranchs first appeared in the triassic strata. 



Fig. 483. — Ventral view of a 

 rather early embryo of Lol- 

 igo; a, arms; e, eye; (7, gill; 

 7i, ear; vi, mantle; s.'sipho- 

 nal folds not yet united; 

 V, vent; y, yolk sac. 



Sub-Class I. — Tetkabrais^ohiata. 



We have incidentally alluded in the preceding pages to some of the characters 

 which serve to separate the cephalopods into two groups, the names of which have 

 reference to the number of gills. The lower of the two, the Tetrabranchiata, are 

 represented by but a single living genus. Nautilus, and from that we have to derive 

 all our knowledge of the soft and perishable portions. These characters may be 



