ANNULOIDA : ECHINODERMATA. 163 



branched processes, which are attached to the extremities of 

 the inter-ambiilacral areas round the mouth ; but others deny 

 that these are perforated by any apertures. These processes 

 are apparently nothing more than greatly-developed tube-feet, 

 and they are probably homologous with the crown of feathery 

 tentacles surrounding the mouth in the Holothurians. 



The sexes are distinct in all the Echinoidea, and the repro- 

 ductive organs are in the form of five membranous sacs, which 

 occupy the inter-ambulacral areas, and open on the exterior by 

 means of the apertures in the genital plates. In the " irregular " 

 Echinoids (such as the "heart-urchins") there are only four 

 genital glands, and, therefore, only four genital plates in the 

 apical disc. .The irregular Echinoids, further, are distinguished 

 by having the anus marginal or sub-marginal, by being mostly 

 destitute of a masticatory apparatus, and by being generally of 

 an oblong, pentagonal, heart-shaped, or discoidal figure. The 

 "regular" Echinoids, on the other hand, have the anus placed 

 at the summit of the test, surrounded by the genital disc ; the 

 test is almost always circular or spheroidal ; and the mouth is 

 armed with a complicated masticatory apparatus. 



The Echinoidea may be divided into the following principal 

 families : 



SYNOPSIS OF THE FAMILIES OF ECHINOIDEA (AFTER POMEL). 



SUB-ORDER I. ECHINIDA. Test composed of no more than twenty rows 

 of plates. 



a. Spatiformes. Mouth excentric, in front; anus behind; anterior am- 



bulacrum obliterated ; form obovate. (The so-called " Spantan- 

 goid" Sea-urchins.) 



1. Ananchytida. With simple ambulacra. 



2. Spantangida. With petaloid ambulacra. 



b. Lampadi formes. Mouth central, or nearly so ; toothed or toothless ; 



anus more or less posterior, but oiten mounting high enough to enter 

 into the genital disc ; ambulacra similar. 



3. Echinoneida. Toothless ; ambulacra simple. 



4. Cassidulida. Toothless ; ambulacra petaloid. 



5. Clypeastrida. Toothed ; ambulacra petaloid. 



6. Echinoconida. Toothed ; ambulacra simple. 



c. Globiformes. Mouth central ; anus opposite the mouth, surrounded by 



the genital plates. 



7. Cidarida. Ambulacra prolonged on the buccal membrane and 



destitute of buccal branchiae. 



8. Echinidce. Ambulacra not prolonged on the buccal membrane, but 



provided with buccal branchiae. 



SUB-ORDER II. PERISCHOECHINID/E OR TESSELATA. Corona of the test, 

 consisting of more than twenty rows of plates. (The Palaeozoic Sea- 

 urchins, Archaocidaris and Palcechinus.] 



