MOLLUSCS. 369 



family, loosely coiled and straight forms being common. One of the most interesting 

 of modern investigations was that conducted by Professor Alpheus Hyatt on the em- 

 bryology of these extinct forms. Every specimen contains in its interior the shell pos- 

 sessed by the embryo, and by breaking open the fossil this can be examined and studied. 

 In this way, by studying large collections. Professor Hyatt arrived at a clear idea of 

 the development of the group, and consequently of the manner in which the numerous 

 genera should be arranged in order to exhibit their affinities. 



Sub-Class II. — Dibkaxchiata. 



In its characters this group differs considerably from the last. As the name 

 implies, but one pair of gills are ]iresent, and as a consequence but one pair of auricles 

 exists The circulation, however, is reinforced by a pair of contractile organs 

 (branchial hearts they are called) on the vessels which convey the blood from all 

 parts of the body to the heart. A pair of kidneys are present. The folds of the 

 siphon are ixnited to form a complete tube ; and the arms which surround the head 

 are furnished with sucking cups. The shell, when present, is internal, and may be 

 either calcareous or horny ; among those with horny shells, it talces the shape of a flat 

 plate or a long style, while in others it may be chambered or flat. An ink bag is 

 always present. This is the group which contains the forms commonly known as 

 squid, cuttle-fish, kraken, pouljic, and the like, and embraces some of the largest 

 animals now living, as well as numerous smaller forms. Thanks to the labors of Prof. 

 A. E. Verrill, the species found on the northeastern coasts of America are as well 

 known as those from any part of the world. The number of arms is made the basia 

 of division into two orders. 



Oeder I. — OCTOPODA. 



In this division the arms which surround the mouth are eight in number, and are 

 covered with sessile sucl^ers in which the horny lip is absent; the eyes are small, and 

 can be covered by the skin, which closes over from all sides ; the body is short and 

 nearly spherical, and lacks either internal or external shell, and usually also the fins so 

 characteristic of most of the decapodous forms ; the mantle is without cartilage, and on 

 the dorsal surface is united to the head by abroad band. The siphon is without valves, 

 and the oviducts are paired in all except Cirrhoteuthis, where the right one is aborted. 



In the CiRRHOTEUTHiD-E a peculiar modification of the arms occurs ; they are united 

 nearly to their tips by a thin membrane, so that the whole corresponds to an umbrella, 

 the eight arms answering to the ribs, while the rather long body bears a fin on either 

 side. The genera are Cirrhoteuthis and Stauroteuthis, and the known species come 

 from the northern seas. Of the latter genus but a single specimen is known ; it is 

 called ,6'. si/rtensis, and was taken about thirty miles east of Sable Island, Nova Scotia. 

 Cirrhoteuthis mulleri is found in the Greenland seas. 



The next family, the Philonexid^, has no fins ; the mantle has an apparatus where- 

 with it may be locked to the siphon ; the .arms are united to a greater or less extent by 

 a membrane, and the dorsal ones are most develo])cd ; there are several pores in the 

 surface of the head which communicate with .aquiferous cavities. In this family hecto- 

 cotylization reaches its greatest extent, the third arm of the right or left {Argonauta) 

 side being thus modified. The auimals swim well. 

 VOL. I. — 24 



