184 REPRODUCTION 



embryo-sac of the ovule. Like the spermatozoa of animals, they 

 are microscopic. In animals each spermatozoon consists of a 

 head which may be elongated, rounded, oval or corkscrew- 

 shaped; it terminates in a long tail or flagellum. The whole 

 of the nuclear matter of the cell is confined to the head, and 

 when once the spermatozoon has penetrated the egg the tail 

 is absorbed. As a rule they are extremely minute, in man 

 only 0-05 mm. in length; but in certain small Crustacea, 

 known as Ostracoda, they attain gigantic proportions. To 

 take one example, in a genus known as Pontecypris they are 

 from 5-7 mm. in length — five to ten times as long as the whole 

 body of the animal. Certain members of the Crustacea 

 have flagellate spermatozoa, i.e. with tails, and this is true 

 of the higher orders of Arthropoda, 

 i.e. the insects, spiders, etc. These 

 flagellae are the only structures of the 

 kind found in the group, and in many, 

 such as the crab and lobster, the 

 spermatozoa are star-shaped and have 

 to be placed in contact with the ova, 

 as they have little power of loco- 

 motion. 



The other large Class of animals Fig. 69. Side view of Cypr 

 whose spermatozoa are devoid of co,ndida, a typical Ostracoda. 

 n ^^ . i j After Zenker, 



flagella are the round-worms, or 



Nematoda. Here they are described as hat-shaped — a de- 

 scription which, considering the present fashions, does not 

 impress a very clear image on the mind. The protoplasmic 

 part of these spermatozoa is amoeboid. 



Some males produce two different kinds of spermatozoa, 

 larger and smaller ; the latter are regarded as the only func- 

 tional forms. The spermatozoa may obtain access to the ova 

 after the latter have been laid, as is the case in many aquatic 

 invertebrates and in most fish, or just as they are being laid, 

 as occurs in lobsters, crabs, and in frogs and toads, or within 

 the body of the mother. This is the case in many worms, 

 leeches, insects, many molluscs and in all the higher verte- 

 brates, reptiles, birds and mammals. 



IS 



