100 SYSTEMATIC ARRANGEMENT 



1. CAECUM. 



Operculum concave externally, smooth. 



I. C. imperforatum, Forbes $• Hartley, B.M. t. K.K. f. 1, cop. 

 Adams, Gen. Moll. t. 38. f. 6. 



Animal of C. imperforatum cylindrical, arcuated. Mantle thick, 

 fleshy. Head long, flat, assisting in locomotion. Mouth vertical. 

 Tentacula short, thick, subcylindrical, setose, slightly clavate. Eyes 

 very minute, sessile, in a line with and a short distance from their 

 base. Neck slender, with two longitudinal ridges from eyes. Foot 

 short, narrow, truncate in front. Gill on left side pale red ; bran- 

 chial leaflets two, one large, the other small, as in Gasteropodan Cana- 

 lifera. Shell with convexity upwards. Operculum circular, horny, 

 black-brown, concave without, with close spiral lines on back of foot. 



C. trachea is the young of C. imperforatum, thrown off as the 

 shell grows. 



Animal marches with great vivacity, carrying the shell sometimes 

 with the convexity upwards, resting on the posterior point or on one 

 of the sides, frequently changing one for the other by suddenly with- 

 drawing the head and body and turning on the side it wishes. 



The foot of Caecum trachea is short, narrow, and truncate ante- 

 riorly, sloping behind to an obtusely pointed termination. Operculum 

 strong, circular, corneous, of seven or eight close-set spiral whorls. 

 The tentacles are elongate, diverging, thickened, and a little spread at 

 the bases. The eyes have decidedly an external bias. The opercu- 

 ligerous lobe is certainly without a cirrhal appendage. — Clark, Moll. 

 518, Append, 



Mr. Alder was only able to examine the jaws and teeth of Caecum 

 trachea very imperfectly ; there are a number of elongated linear 

 curved teeth, which appear like the outer lateral ones. 



The animal of Caecum pulchellum of New England, found adhering 

 to Vermeti, agrees nearly with the English species C. trachea, as 

 described by Mr. Clark. The head projects a little in advance of 

 the foot, which is short. The muzzle is cleft and transversely wrin- 

 kled, and has two black spots above just in front of the tentacula, 

 which are thick, curved, and covered with large vibrillse. The eyes 

 are conspicuous, black, oval, and situated at nearly the middle of the 

 bases of the tentacula, a little towards the inner sides. The oper- 

 culigerous lobe projects a little beyond the operculum. The oper- 

 culum multispiral, of almost eight volutions, corneous, and concave 

 on the outer surface. The shell in its adult state is clavate, arcuated, 

 contracted at both extremities, and having a somewhat angular ap- 

 pearance at its outer or dorsal outline, thick and strong, with strong 

 rounded ribs, broader in front. 



Mr. Stimpson observed, while keeping this species alive from 

 April to November, that it presented three stages of growth : first, a 

 slender, thin, arcuated form, with few distant ribs ; secondly, the an- 

 terior half of the former form left by the decadence of its posterior 

 half, with a part of the growing adult shell ; and thirdly, the adult 

 shell, which is clavate, arcuate, contracted at both extremities, and 



