102 COMPARATIVE ANATOMY. 



pronounced at maturity. (2) Special characters are those pe- 

 culiar to times of procreation and rearing of young. 



(1) Size. With Invertebrata the female is the larger; with 

 the Vertebrata the reverse is true as a rule. In species where 

 the female carries large numbers of eggs within her tissues, 

 either preparatory to ovi-position or throughout development, 

 their accommodation and support necessitate extensive growth 

 and active nutrition. Such females are always larger than the 

 corresponding males. Examples are found in many nematoid 

 Entozoa, some rotifers, as in Asplanchna, in Argonauta (paper 

 nautilus), and in many insects, as particularly seen in Termes 

 (termite ant).* 



Special organs. Special organs may be divided into those 

 (a) for the attraction of the opposite sex, such as luminous 

 organs of Photinus scintillans (small firefly) ; vibratory, stridu- 

 lous organs, as in Gryllus (cricket), where certain nervules of 

 the anterior wings rasp against one another ; or, as in Cicada 

 (American locust), where a cavity placed upon either side of 

 the last thoracic segment is covered in by a tympanic mem- 

 brane which is rendered tense and vibratory by muscular 

 action; (b) for retention of the female in coitus, such as the 

 claspers in Squalus (shark), where they are placed one on 

 either side of the position of the genital pores; or in the 

 sternal plates and finger-like appendages to front feet, as in 

 a genus of Batrachia (Cystignathus)-, (c) other characters, as 

 spur of Gallus (cock), and Ornithorhynchus (duck-bill). Pecu- 

 liarities of existing structures, as seen in growth of hair on pubes 

 and face (man), on neck (lion) ; or in modifications of voice 

 due to changes not fully appreciated. 



(2) Increased functional activity: as in pigment cells of ova- 

 ries of Toxopneustes (sea-urchin), in color of fins in Cypri- 

 noid fishes, $, in plumage and wattles of birds, $ 9: as 

 in secretion of the cervical gland of Crocodile, 1 , sebaceous 

 gland of penis of Anser (goose), fc , labial gland of Camelus 

 (camel), $ ; testicles and ovaries of Aves, and in the "moult- 



* In some animals, us certain Cynipidce (gallflies), and some entomostracan 

 crustaceans, the male is unknown. 



