THE AMPHIPODA 237 



majority of the Hyperiidea (Fig. 136, 0) they are enormously 

 enlarged, occupying nearly the whole surface of the head. In this 

 case each eye is commonly divided into two parts, the dorsal 

 differing in the great elongation of the crystalline cones and in 

 other details from the lateral division. Apart from the Am- 

 peliscidae, the eyes of the Amphipoda are characterised by the 

 fact that the corneal covering is not faceted, or, in other words, 

 that corneal lenses are not formed. 



In addition to " sensory filaments " of the usual type borne 

 by the main flagellum of the antennule, and sometimes particularly 

 numerous on the enlarged proximal segment of the flagellum 

 (Lysianassidae, Hyperiidea), many Gammaridea have on the 

 antennae, and sometimes on the antennules, peculiar organs known 

 as calceoli. These are, in the simplest cases, little flattened vesicles 

 attached by a narrow stalk, but in some the structure is more 

 complex. They are often, but not always, confined to the male 

 sex, and have been variously interpreted as olfactory or auditory 

 organs, as adhesive suckers, and even as sexual ornaments. 



A pair of statocysts have been described in connection with the 

 anterior part of the brain in Oxycephalidae. 



Reproductive System. The paired ovaries and testes are of 

 simple tubular form and lie in the thoracic region. The testes 

 are continued posteriorly into short vasa deferentia which may be 

 slightly dilated into seminal vesicles and open on short papillae 

 on the sternum of the last thoracic somite. The oviduct leaves 

 the ovary at about the middle of its length and opens on the inner 

 surface of the fifth coxal plate (sixth thoracic somite). It is stated 

 that an actual opening does not exist until the moment when the 

 eggs are extruded. The spermatozoa consist of a slender, rod-like 

 head, to which a filiform tail, stated to exhibit vermiform move- 

 ments in some cases, is attached at an acute angle. Definite 

 spermatophores are not formed, at least in the majority of Amphi- 

 poda. The spermatozoa are deposited on the ventral surface of the 

 body of the female immediately before the eggs are extruded, and 

 fertilisation is external. The occurrence of ova within the testis 

 has been observed in species of Orchestia, where it is perhaps 

 universal in young individuals. 



DEVELOPMENT. 



Segmentation is at first total, later becoming superficial, with 

 early differentiation of the blastoderm on the ventral side. The 

 teloblastic mode of growth in the post-naupliar region of the 

 embryo which occurs in many Isopoda and Mysidacea does not 

 present itself. 



A " dorsal organ " is early developed as a median thickening 



