368 



LOWER INVERTEBRATES. 



biiefly enumerated as follows : The two siphonal folds are never united ; the tentacles 

 are numerous and never bear suckers ; the gills are four in number, and, corresponding 



to them, four auricles are present ; the renal organs 

 are also in two pairs, and branchial hearts do not 

 exist. The character of the eye has already been 

 described. The shell is either straight or curved, 

 never internal, and is divided into a series of cham- 

 bers. Two well-marked families exist, but lately 

 they have been broken up into a large number of 

 groups, to which family rank has been accorded, but 

 with these we need not concern ourselves. 



In the Nautilid^, the partitions forming the 

 chambers of the shell ^re simple, and concave on 

 the outer surface ; the siphuncle occupies a nearly 

 central position ; the aperture is simple, and the 

 Fig. 485. - Under s^e of a part of a Nau- external Surface is nearly or entirely smooth. Over 

 vent-foTiducte!'*""""'^*'"'^"^' "' two thousand species have been described, of which 



only six are living at the present time, all belong- 

 ing to the genus Nautilus, in which the whorls of shell are few, but closely coiled 

 in a flat spiral. The shells are very common, but the animals are rarely seen ; 

 this may partly be due to the habits, but more from the fact that the natives of 

 the South Seas eat the animal when obtained, but use the shell for barter. These 

 shells are composed of two layers, the outer one being dull and opaque, but the inner 

 one is pearly, whence all the species derive the common name pearly nautilus. Speci- 

 mens converted into cameos by removing parts of the outer layer by means of acid 

 are very common, some being really artistic ; this point, of course, depending entu-ely 

 on the skill of the artist. The Nautili live in comparatively shallow water, where 

 they creep slowly over the bottom, or swim by means of the water forced out of the 

 siphon. They live on animal matter, and the traps in which they are caught are 

 baited with crabs, searurchins, and the like. 



Of the many fossil genera we need say but little. They occur in all the rocks from 

 the lower Silurian to the present time. In 

 shape, all transitions may be found, from the 

 perfectly straight Orthoceras to flat spirals like 

 Nautilus, and conical spirals like Trochoceras. 

 Some of the species of 

 the former genus grew 

 to an enormous size, 



and Prof. J. S. Newberry estimates that the fossil Orthoceras 

 titan weighed some tons. 



In the Ammonitid^ the septa are lobed or folded at their 

 margins, so that the line of their insertion into the shell fre- 

 quently has a bushy or dendritic appearance. Associated with 

 the Ammonites frequently occur horny or shelly plates called 

 aptychi or anaptychi, the relations and functions of which are 

 ^m.m— Ammonite. entirely problematical. None of the family are now living, 

 and but a few extended into the tertiaries. The bulk of the species belonged to the 

 mesozoic rocks. In form they vary almost exactly as do the members of the last 



Fig. 486. — Ancyloceras. 



