244 



LOWER INVERTEBRATES. 



whole world, is relatively small, not over fifty, of which North America possesses about 

 a dozen. The names which the different genera have received are more musical than 

 in some other groups, owing to the diminu- 

 tive "ella," with which most of tliem ter- 

 minate. 



The CmsTATELLiD-E are readily distin- 

 guished by their free condition. They form 

 large colonies, the individuals being arranged 

 in concentric circles or ovals, on the upper 

 surface, while the lower is modified into a 

 contractile fleshy foot, of use to the colony 

 Two s])ecics 

 this country, and one 

 Euroj)e, both belonging to the genus C'ristutella. 



The family Plujiatellid.e, which is much larger, embraces sessile 

 forms, the various genera of which are distinguished by the gelatinous or parchment-like 

 nature of the cells, the structure of the statoblasts (with or without spines), and the 

 massive or branching nature of the colony. Four genera, Fredricella, Plumatella, 

 Ziophopus, and Pectinatella, are represented in the waters of eastern North America. 



Fig. 246. — Cypkmiautes, young . . , , 



of Membrauipora ; m, mouth; m its slow, crecpmg motion. 

 s, stomach. , • ^i • 



are known in this countrv. 



Sub-Class III. — Podostomata. 



The genus Ixhabdopleara so differs from the other Polyzoa as to warrant the erec- 

 tion of a sub-class for its reception. It approaches most closely of all the class to the 

 Mollusca. It consists of a creeping root-stalk, of a chitinous nature, from which arise 

 the tubular branching cells. Each cell has a round terminal mouth, and the walls of 

 the cell are annulated for some distance below the mouth. The various cells of the 

 colony are separated by transverse partitions. The long arms of the lophophore bear 

 two series of tentacles, and resemble somewhat those of the Phylactolsemata, but 

 much more closely those of the brachiopods. The animal is fastened to its cell by a 

 long, contractile filament, by which it draws its body do.wn out of the way of harm. 

 When the danger is past, according to Sars it literally climbs out of its tube, by means 

 of a disc between the arms, which appears to represent the epistome of the fresh- 

 water forms. Cephalodiscus is the only other genus of the sub-class. It was dredged 

 by the 'Challenger' expedition. 



Class II. — BRACHIOPODA. 



The shells of the brachiopods, at first sight, closely resemble those of the lamelli- 

 branch molluscs, and hence it is not strange that these forms were for so long a time 

 associated together. Indeed, even at the present time most geologists fail to recognize 

 the important differences, a fact probably due to their ignorance of the anatomy and 

 embryology of the living forms. 



The brachiopods, which are all marine, are jtrovidcd with a bivalve shell, but the 

 two valves of the shell are always dissimilar, while the two sides of each valve are 

 alike, just the reverse of what obtains among the lamellibranchs. In the chemical 

 structure of the shell, also, an important fact is to be noted, that phosphate of lime is 

 present in much larger proportion than in that of any true mollusc. Most of the 



