AND ITS ALLIES. 213 



philodinoid shape. The central segments form a broad, barrel- 

 shaped or almost quadrate expansion, which is marked off by 

 deep constrictions from the narrow extremities, both of which 

 taper to very slender terminal segments. 



The slender rostrum and foot give a certain resemblance 

 to the animals of the genus Rotifer, which are also viviparous. 

 Against these points of correspondence must be set the posses- 

 sion of four toes, of cervical eyes (when any), and of only 

 four foot-joints, instead of the five of most, if not all, species 

 •of Rotifer. 



The constriction anterior to the broad central trunk separates 

 a section of the body, still regarded as part of the trunk 

 (see Figs. 1, 2, 8, 12, etc., on Plates 15 and 16). The central 

 and anterior parts of the trunk are longitudinally plicate, the 

 plicae in the centre of the back broad and shallow, getting 

 narrower and deeper towards the sides. The ventral surface 

 of the central trunk is obscurely transversely plicate, in one 

 variety of P. aculeata distinctly so. 



The trunk is thick-skinned, rarely hyaline and colourless — 

 more generally more or less yellowish, in consequence of a 

 secretion from the skin, which may cause extraneous matters 

 to adhere, and which often of itself forms a thick casing of 

 plates. The pores which produce the secretion give the surface 

 a strongly stippled appearance, often quite papillose. 



In front of the trunk come the three thinner-skinned neck 

 segments, diminishing in size forwards, the third carrying the 

 antenna. 



Then follows the oral segment, forming in the creeping 

 attitude, by reason of its bulky contained organs, an expansion. 



The oral segment is abruptly contracted to the narrow two- 

 jointed rostrum. 



The lower joint of the rostrum is a sheath for the upper 

 joint as usual. The terminal joint is slender, and has its tactile 

 armature very well developed (see PI. 17, Fig. 18). 



The tactile organs are of four kinds — the lamellae, the brush 

 of vibratile cilia, the straight setae, and the large tactile setae. 



The tip of the rostrum is invertile, and all those organs can be 



