358 



LOWER INVERTEBRATES. 



swim in a very eri'atic manner, darting hither and thither with gi-eat rapidity, turning 

 suddenly, and being always on the alert for food. They, like all the rest of the group, 



are carnivorous, devouring immense quan- 

 tities of microscopic animals. The species 

 of Cleodora swim in a much more leisurely 

 manner, not turning as abruptly as those 

 of the former genus. They have a straight, 

 triangular shell, terminating in a sharp 

 point behind, and in front with a trian- 

 gular mouth. In Styliola (or Creseis) the 

 shell is round and needle-shaped, running to 

 an acute tip. The species average about 

 half an inch in length and have perfectly 

 transparent, glassy shells, through which all 

 parts of their internal structure can be dis- 

 tinctly seen. S. vitrea occurs on the New 

 England coast. Cuvieria is another genus 



Fig. 468. — Hyalea trideiitata. i_ • i_ i 1 1 ■ . t ■, 



Which should receive mention. In early 

 life it is much like Styliola, but with growth it partitions off the 

 hinder parts of the shell, and the body moves forward and builds a 

 swollen and more nearly cylindrical shell. The deserted portions 

 of the shell soon become broken off, and the result is the truncated 

 appearance shown in our figure, which illustrates a species common 

 to the Mediterranean and the tropical Atlantic. I have seen numer- 

 ous specimens, each about half an inch in length, collected between 

 the Bermudas and Florida. 



Here, in all probability, belong the problematical fossils known 

 as Conularia or ' cone-in-cone.' They first appear in the Silurian, 

 and some reach, for pteropods, an enormous size, an Australian 

 species being estimated to have had a length of about sixteen inches. 

 They have a four-sided shell, with the apex partitioned off by nar- 

 row, closely placed septa, so that the whole resembles a series of 

 cones, or rather of pyramids, nested within one another. Those 

 other long and slender fossils, ornamented with rings, and known Tentaculites, should 

 also be placed here. They occur in Silurian and Devonian rocks. 



The LiMACiNiD^ are readily recognized by the spirally-coiled shell, the whorls of 

 which are sinistral. The mantle is large and opens dorsally. The species are largely 

 inhabitants of the colder waters, some being found in the Arctic and Antarctic seas. 

 On our coast a member of the family, S2nrialis gouldii, is not very uncommon in the 

 evening, both north and south of Cape Cod. Mr. Alexander Agassiz kept several in 

 confinement for a while, and observed that during the day they were quiet, and rarely 

 left the bottom of the jars in which they were confined, but after dark they became 

 very active and arose to the surface. 



The Cymbulid^ are noticeable for their comparatively large size and the very 

 peculiar shell which they secrete. In early life, like the rest of the group, they have 

 a small, spiral, horny shell, but this becomes lost and in its place the animal secretes a 

 cartilaginous slipper-shaped shell, apparently possessing no more consistency than 

 ordinary gelatine jelly. In this thick, transparent, flexible shell sits the mollusc, like 



Fro. 469. —Cleodora 

 pyramidata. 



