354 CRUSTACEA. 



larva (and, in Cumacea, and Stomatopods in the adult). In 

 the Entomostraca the latter is usually the functional gland in 

 the adult. It opens at the base of the second maxilla, and is 

 frequently contained in the fold of the shell, and hence known 

 .as the shell gland. 



Reproductive Organs. The sexes are distinct in the great 

 majority of the Crustacea. The Cirripedia, however, which 

 lose their locomotive powers early in larval life and become 

 attached, are in nearly all cases hermaphrodite, thus offering 

 a parallel with many, though by no means all, other sessile 

 animals (Tunicata, Phoronis, many Polyzoa). Among Cirripedes 

 also occurs (Scalpellum, Ibla) a remarkable form of dimorphism, 

 the association of supplemental and degenerate dwarf males 

 with the hermaphrodite individuals (p. 424). Several genera of 

 the parasitic Cymothoidae and Epicaridea (Isopoda) are also 

 hermaphrodite, and protandrous, i.e. a male phase precedes the 

 female phase in the life of the individual (p. 484). The 

 remarkable tendency to a hermaphrodite condition in male 

 Decapods, when they recover from the suppression of the re- 

 productive organs induced by cirripede and other parasites is 

 alluded to below (p 445). 



The reproductive organs are formed on the same type in the 

 two sexes, and usually occupy similar positions, dorsal to the 

 intestine. In both they consist of a single pair of gonads 

 with the corresponding ducts, though fusion to a greater or 

 less extent between those of opposite sides may occur. The 

 ducts (mesodermal) are continuous with the gonads, and meet 

 distally an invaginated ectodermal tract opening to the 

 exterior. 



In Cyclops the rudiments of the gonad (" stem-cell ") can be 

 distinguished at an early stage of segmentation. 



The spermatozoa vary widely in character. They are amoeboid 

 in Polyphemus (Cladocera), oval or sausage-shaped in the 

 Copepods, spherical and beset with stiff radial processes in some 

 Decapods, Euphausiidae and Stomatopods. They are often 

 filiform (though not always, e.g. Danalia) in Isopods and Am- 

 phipods, in the Cirripedes, and the Ostracods, in which latter 

 group they are of gigantic size (up to 10 lines), exceeding the 

 whole body in length three or four times. In many cases they 

 are apparently not mobile, but it is not the case, as sometimes 



