162 ANIMAL KINGDOM. 



the anal opening of a Crinid is situated on one side of the body, it being lateral, 

 as in the Echinoid in fig. 157 ', and the intermediate group of plates numbered 

 10 are called the anal. 



Nearly all the Palaeozoic Crinoids have the anal and oral apertures together, 

 there being but a single opening in the summit ( Cyathocrinus is said to be an 

 exception). This opening is sub-central (fig. 524) or lateral (fig. 525), and 

 in the former cases is often (as in many species of Actinocrinus) situated at the 

 summit of a slender proboscis made up of small pieces and sometimes three or 

 four times as long as the body. In some species of Poteriocrinus (P. 3fis- 

 souriensis Shumard = P. longidactylus, Shumd. Mo. Rept., p. 188) the pro- 

 boscis is very large, being nearly as wide as the body and four or five times 

 as long. It is composed of regular ranges of hexagonal plates, with a series 

 of pores between every alternate range, much like the ambulacral pores in the 

 true Echinoids. 



In the Cystids the aperture is generally lateral and remote from the top, as 

 in fig. 156, while the arms come out often from the very centre. 



The Cystids are also peculiar in what are called pectinated rhombs (see fig. 

 156) ; that is, rhombic areas crossed by fine bars and openings : the use of them 

 is uncertain, — though they are probably connected with an aquiferous system 

 and respiration. The Cystids are the most anomalous of Radiates. 



Among the Crinidea there are — first, the species with arms and rounded stems, 

 which are of several genera and families. Second, the Pentacrinus group, which 

 also have long arms, but the stems are five-sided. Third, the Blastoids, which 

 have round stems, but the body is pentagonal and flower-like or petaloid in its 

 divisions, and the divisions are furnished with pinnules : Ex., the Pentremites. 

 As they are usually found closed up, they have a resemblance to a flower-bud ; 

 and hence the name, from the Greek. 



2. Acalephs. — Besides the jelly-like Acalephs, which have very 

 rarely left any traces in the strata, there are delicate coral-making 

 species of the Hydroid group. Fig. 151 represents a Hydra, 

 much enlarged; 152, a related animal of the Tubularia family 

 (genus Syncoryna). Other species, having animals like 151, as in 

 the genera Campanularia and Sertularia, form very delicate mem- 

 branous coralla, which under the microscope consist of series 

 of minute cells ; and the fossils called Graptolites have been com- 

 pared to them. The hard, stony corals called Millepores have 

 been shown by Agassiz to have animals like fig. 152, and there- 

 fore to belong to the class of Acalephs. The genera of fossil 

 corals Chcetetes and Favosites, having the cells divided by horizontal 

 partitions, and being in this respect like Millepores, he refers to 

 the same group. 



There are, hence, not only stony corals made by Polyps, as in the 

 common kinds, but there are also large stony corals made by Aca- 

 lephs, besides delicate kinds which were made either by Hydroid 

 Acalephs or Bryozoan Mollusks. 



