494 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the common jellyfish (Aurelia, cf. page 541); the loss 

 by reflex action on irritation of an arm is paralleled by 

 the tail of the lizard ; while the formation of buds by 

 the injured arm or disc reminds us of the capacity for 

 reproduction which is exhibited by the common 

 Hydra. 



Asexual reproduction in Echinoderms may, to 

 sum up, be one of two things ; there may be fission 

 with repair, or a single arm may be separated off 

 and give rise by external gemmation to a fresh zooid. 



It is of interest to note that these phenomena are 

 seen especially in those forms (Linckia) in which the 

 ossicles of the arms are not protected either by strong 

 spines as in Astropecten, or by well-developed margi- 

 nal plates as in Pentagonaster. 



A high grade of differentiation is reached by some 

 of the Arthropoda, for the germinal glands are 

 compact, and often, indeed, united into a single mass ; 

 the efferent ducts are not unfrequently single, and 

 their orifice median in position, while the distinction of 

 the sexes is often to be demonstrated by the posses- 

 sion of secondary characters, such as differences in 

 external configuration, or the characters of certain 

 pairs of appendages. It is but seldom that the sexes 

 are united in the same individuals, and the mode of 

 life ordinarily allows us to find an easy explanation 

 for this arrangement, when it does obtain. 



On the other hand, there are some remarkable ar- 

 rangements obtaining in various Arthropods which 

 require immediate notice ; in the Crustacean Apus, 

 and in such insect forms as the bee (Apis), and the 

 plant-louse, Aphis, we find that for a large number of 

 generations the females are enabled to produce ova, 

 which grow up to the adult stage without the inter- 

 position of any male influence ; here, too, as in the 

 case of the Rotifera (see page 482), the presence of 

 polar globules in the maturing egg-cell have not yet 



