430 



HISTORICAL GEOLOGY. 



2. The Blastoids {Pentremites , etc.). — Having a symmetrical ovoidal body, with 5 

 petal-like ambulacra meeting at the summit, without proper arms, and attached by a stem 

 like that of the Encrinites. 



3. The Cystoids (from the Greek for a bladder), Fig. 444. — Arrangement of the plates 

 not often regularly radiate. Arms, when present, proceeding from the center of the sum- 

 mit instead of the margin of a disk ; in some, only 2 arms ; in others, replaced by radiat- 

 ing ambulacral channels, which are sometimes fringed with pinnules. 



In ancient Crinoids, the arms are not generally free down to the base, but there is a 

 union of their lower part, either directly or by means of intermediate plates, into a cup- 

 shaped body or calyx (as in Fig. 443, and also Figs. 995, 999, under the Subcarboniferous 

 period, page 640). 



In Fig. 434, the plates of one of these cups, in the species Batocrinus longirostris 

 H., are spread out, the bottom plates of the cup being at the center. The plates, it is 

 seen, are in 5 radiating series, corresponding to the 5 rays or arms of the Crinoid, and 

 between are intermediate pieces. The 3 plates numbered I are called the basal, as the 

 stem is articulated to the piece composed of them ; 3, 3, 3 are the radial ; 4, 4, supra- 

 radial ; 5, brachial, situated at the base of the arms ; 7 are immediate plates, called inter- 

 radial ; 8, another intermediate, the inter-supraradial. Sometimes, in other Crinoids, 

 there is another series of plates, at the junction of the plates I and 3, called sub-radial. 

 Finally, the anal opening of a Crinoid is situated toward one side of the disk, it being 

 lateral , as in the Echinoid in Fig. 432 ; and the intermediate group plates numbered 10 

 are called the anal. 



In the Cystoids, the aperture is generally lateral and remote from the top, as in Fig. 

 444, while the arms often come out from the very summit. The Cystoids are also peculiar 

 in what are called pectinated rhombs (see Fig. 444) ; 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. 



445. 



6. Coelenterates. 



The Coelenterates are distinguished from Echinoderms by the existence of only one 

 opening to the digestive system, the mouth. Moreover, the tentacles and other parts are 

 never normally a multiple of 5, but either of 4 or 6 ; of 4 in Hydrozoans and 4 or 6 

 in Polyps. 



1. Hydrozoans (Acalephs, Medusae, Jelly-fishes, Hydroids). — Having the body, in the 

 adult stage, usually nearly transparent or translucent, looking jelly-like ; and internally a 



stomach-cavity, with radiating branches. Ex., the 

 Medusa, or Jelly-fish (Fig. 438), which generally 

 floats free, when in the adult stage, with the mouth 

 downward. The Hydra and allied species are 

 here included. Most mai'ine Hydroids at times 

 produce sexual buds, which, in many species, break 

 away and become free jelly-fishes. 



Many of the Hydroids make corals, and hence 

 are common as fossils. Fig. 439 represents a Hydra 

 enlarged, with a young one budded out from its 

 side. Some species of the group — those of the 

 Sertularia tribe — form delicate chitinous corals, 

 such as are represented in Fig. 445, in which each 

 notch on the little branchlets corresponds to the 

 cup-shaped cell from which an animal protrudes 

 its flower-shaped head. (a is the Sertularia 

 abietina ; b, S. rosacea; and a', b', portions of branches enlarged.) The interior cavities 





Hydrozoans. Figs, a, a', Sertularia abie 

 tina; b, b', 8. rosacea. 



