VII] LIFE HISTORY 125 



produce females; the smaller eggs, whose origin seems to be deter- 

 mined by the temperature, may amount to eighteen or twenty. 

 These produce the males. Towards the autumn the males fertilise 

 the females, and the resulting eggs termed "winter-eggs" are 

 clothed with a thick shell capable of withstanding cold and drought. 

 These live through the winter and give rise to females in the 

 spring. A similar alternation of summer and winter eggs is met 

 with in certain Crustacea (Phyllopoda and Ostracoda). 



Eotifera are cosmopolitan, but as a rule they inhabit fresh water ; 

 about 700 species are known, of which only one-tenth live in the 

 sea or in brackish water. One species, Synchaeta baltica, is pelagic 

 and phosphorescent. A few are parasitic. Hydatina senta is one 

 of the commonest Rotifers, and is usually to be met with swimming 

 about amongst the algae of green ponds. It possesses a slightly 

 elongated, cylindrical body (Fig. 53). The disc bears a circular 

 cingulum separated by a groove from the trochus ; in this groove are 

 five prominences bearing stiff setae. The posterior end of the body 

 tapers and ends in a bifurcated foot on which a pair of glands open. 



A great many of the Bdelloida live amongst the roots and 

 leaves of mosses, etc., and these can survive being dried up for a 

 long time, the body shrinking and sealing itself up in the cuticle. 

 Apart from the species which possess this power Rotifera as a class 

 are short-lived, and this is especially true of the males. 



In certain respects, such as the nature of their excretory organs 

 and of their ovaries, the Rotifers show some resemblance to the 

 Platyhelminthes ; but they differ from them profoundly in the 

 alimentary canal. The velum, in its typical form consisting of 

 trochus and cingulum, has been compared to the ciliated baud 

 which encircles the Trochophore larva of Annelida of which more 

 will be related later. This band like the velum is seen on close 

 inspection to consist of a prae-oral and a post-oral loop. The 

 Trochophore larva is found in the life-history of the division of 

 Annelida termed Polychaeta and of some of the more primitive 

 Mollusca, and it is believed by many to represent a common 

 ancestral form. Should the comparison of the Rotifera with this 

 larva be a just one, we must regard the Rotifera as having been 

 derived from the common stock of Annelida and Mollusca and to 

 be therefore a very ancient group. There are however difficulties 

 which arise when the comparison is carried into details, so for the 

 present it is better to regard the Rotifera as a completely isolated 

 phylum. 



