MOLLUSCS. 



367 



capsules arc united in a bunch, but I have seen one muss of the eggs of tliis species 

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

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

 modified from the same reason. 



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

 known throws but little light on the relationshijts 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 ai-e very like those in the chick. The blastoderm 

 gradually increases until it envelopes 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 develojjment progresses they take the form of two dis- 

 tinct flaps, a condition which is permanent in Ncmtilus, and 

 then the edges fuse together. The first ap|)earanee of the arms 

 antlers 'si phi ^* ^" ''^"^ shape of simple jn-ominences. 

 S,''TOn°tf',,"ioiulic';'°"^''' ^* t^"s time the yolk extends as a huge 

 mass from between the arras, but, as 

 growth 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 

 Molhisca, 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. Tliis tendency exists everywhere in the animal king- 

 dom, but of course it requires time to eliminate them all. 

 Time the cephalopods have had. In the lower Silurian rocks, 

 fossil forms not generically distinguishable from living Nctti- 

 tili are found, and the tetrabi-anchs flourished in palfeozoic 

 time, while forms assigned to the dibranchs fii'St appeared in the triassic strata, 



Fig. 483. — Ventral view of a 

 rather early embryo of Lot- 



Fig. 484. — Embryo sqviid in 

 whieh the si phonal folds (.s) 

 have united, an<l the suck- 

 ers or acetabula are .ap- 

 pearing upon two pairs of 

 the arms; a. arms: c, eye; 

 /, fin; A, ear; r, vent. 



Sub-Cla.ss I. — Teteabkanchiata. 



We have incidentally allu<lod in the preceding pages to some of the characters 

 which serve to separate the cei)haiopods 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, N<tvtili(s, and ffom-_that we have to derive 

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



