578 THE ANIMAL KINGDOM. 



respiratory in function. A single or double aperture lies at the apex of 

 each oral or deltoid piece. These are the spiracles or efferent external 

 apertures of the hydrospires. The anus is confluent with one of them. 

 The genital ducts probably opened into some portion of the hydrospires, 

 and the spiracles in that case discharged the genital products. A circum- 

 oral ring with five radiating tubes, one under each lancet piece, represents 

 the water-vascular ring and ambulacral tubes. 



In some American forms a vault of minute plates, without definite 

 arrangement arches over the mouth and spiracles, except in Elecrinus, 

 where there is an oro-central and five orals. A series of covering plates 

 may be continued down the ambulacral area concealing the groove. This 

 arrangement is identical with what obtains in many Palaeo-crinoidea. 



The Blastoidea appear later than the Crinoidea and Cystoidea in the 

 Upper Silurian, and reach their greatest development in the Devonian and 

 Carboniferous strata in which they die out. 



Carpenter (P. H.) and Etheredge, A. N. H. (5), ix. 1883 ; Carpenter, op. cit, 

 (5), xv. 1885. 



VERMES. 



The classes and minor groups collectively termed Vermes do not con- 

 stitute a phylum in any way comparable for example to the phyla Mollusca 

 or Echinodermata. It is easy to show that the various Molluscan and 

 Echinoderm classes are derived by a modification of a set of well-defined 

 and peculiar characters. There is consequently amid much diversity a 

 common thread running through the series. On the contrary the types of 

 structure seen in most Vermian classes are very distinct from one another ; 

 they are specialised and therefore genetically remote. Attention never- 

 theless may be directed to certain general features. 



The antero-posterior axis is usually of considerable length, the trans- 

 verse and dorso-ventral short ; but in the Myzostomidae and a few Polyclad 

 Turbellaria the antero-posterior and transverse axes tend to an equality, 

 and hence the body is disc-like. A ventral aspect is always distinguishable 

 from a dorsal except in Acanthocephala ; it is usually characterised by being 

 flattened for locomotor purposes : sometimes also by the presence of various 

 apertures, e.g. mouth, anus, &c., or of the main nervous cords. Bilateral 

 symmetry is well marked, but a more or less apparent radial symmetry is 

 observable in some Polyclad Turbellaria^, A division of the body into 



1 Much stress has lately been laid on the Ctenophoran characters of certain Polyclad Tur- 

 bellaria, The presence of a free dorsally placed mass of otoliths in two genera, and of Ctenophore- 

 like ciliary plates in one of the two, are striking features. But the existence in Turbellaria of a well- 

 developed excretory system, the confinement of the principal nerve-cords to a ventral plane even 

 when their radial symmetry is most marked, are features decisive against any close alliance with 

 Ctenophora. 



