194 



INVERTEBRATE MORPHOLOGY. 



of generations reproducing by these so-called " summer ova " 

 may thus succeed each other without the intervention of a 

 male. Under certain conditions, however, certain females pro- 

 duce " summer ova " of a smaller size than usual, which, devel- 

 oping parthenogenetically, give rise to the males. In addition 

 to these two forms of " summer ova," some species produce a 

 third kind of egg, the so-called " winter ovum," which differs 

 from the summer ova in containing more yolk and in being 

 enclosed within a stout resistant shell. It seems probable 

 that these ova develop only after fertilization. 



There are two Rotifers which deserve a special description on account of 

 their having served as a basis for phylogenetic speculation. One of these, 



Trochosphcera (Fig. 94), is spherical 

 in shape ; a band of cilia runs round 

 the equator of the sphere, not encirc- 

 ling it completely, however, but leav- 

 ing an unciliated region on the dorsal 

 surface. Anteriorly this band passes 

 above the mouth-opening, which is 

 bounded below by a very small post- 

 oral band and opens into a pharynx 

 provided with a mastax (Ma), from 

 which the stomach, with digestive 

 glands, passes towards the centre of 

 the body and there bends at right 

 FIG. 94. TrocfwspTmra wquatorialis angles to open through the intestine 



11) U 



(after SEMPER). 



A anus. 



Cl = cloaca. 

 Ex excretory tube. 



M = mouth. 

 Ma = mastax. 

 mu muscle. 



N = uerve-ganglion. 



n = nerve. 

 o = ovary. 



So = sense-organ. 



into a cloaca (Cl) which receives the 

 excretory tubules (Ex) and the oviducts 

 and opens to the exterior at the lower 

 pole of the sphere (A). The brain (N) 

 lies above the pharynx and sends 

 nerves to the two eyes situated, one 

 on each side, below the equatorial 

 band of cilia, and also to a small 

 sensory papilla (So), probably the 

 calcar, lying on the dorsal surface. 



this nerve () encircling the anterior 

 half of the sphere, and running in a plane at right angles to that in which 

 the ciliated band lies. 



The other form belongs to the genus Hexarthra and differs from other 

 Rotifers principally in the occurrence of six hollow processes of the body, 

 arising from the ventral surface and arranged in pairs diminishing in size 

 from before backwards. Each is terminated by a bunch of stiff bristles or 

 setae, and all are supplied with muscles whereby they can be rapidly swept 



