OMNIVOROUS ECHINODERMS, &c. 



265 



these being due to a series of short branches (pinnules) with 

 which each of them is beset on either edge. The Feather-Star 

 is commonly found moored to some firm object by a circlet of 

 filaments attached to the back of the central disc. The motdh 

 is situated in the centre of the body, and from it ciliated food- 

 grooves run along the arms, sending branches along the pinnules. 

 The currents set up by the action of the cilia flow along the 

 grooves to the mouth, into which they carry all sorts of minute 

 organisms and other particles serving as food. 



The other members of the Crinoidea are Sea- Lilies (see vol. 

 i, p. 460), which live in the deep sea, and may be compared to 

 Feather- Stars attached to various objects by long stalks. Their 

 way of feeding is precisely the same. 



Passing over the members of the phylum Ccelenterata, 

 which in typical cases are actively carnivorous (see p. 155), we 

 come to the two low- 

 est groups in the 

 animal kingdom, i.e. 

 Sponges and Proto- 

 zoa, the species in- 

 cluded in which are 

 mostly or largely 

 omnivorous. 



SPONGES (PORIFERA) 



Sponges are either 

 simple or colonial 

 animals of sluggish 

 habit, which live at- 

 tached to stones or 

 other firm objects. 

 The structure of a 

 simple Sponge has 

 elsewhere been de- 

 scribed (vol. i, p. 484), 

 and it need only be 

 stated here that the 

 body is vase-shaped, with its walls perforated by numerous holes. 

 The lining of the vase is composed of a layer of remarkable 



- -trv 



Fig. 471.— Vertical Section of Simple Sponge (enlarged and diagrammatic) 

 ec, External body-layer; m, middle body-layer, with spicules, s; en, inter- 

 nal body-layer, made up of flagellated cells and extending into the canals [vj], 

 which pierce the side-wall of the body, opening into the central cavity, g, 

 that also communicates with the exterior by a large aperture, a. The arrows 

 indicate the course taken by the currents of water which traverse the body. 



