194 THE CRUSTACEA 



in the male ; the chelae are often much stronger and differently 

 shaped ; and the pleopods are always well developed in that sex 

 even when they are reduced or absent in the female. In several 

 genera of Tanaidae the oral appendages, with the exception of the 

 maxillipeds, are entirely lost by the sexually mature male. Fritz 

 ]\Iiiller described a remarkable dimorphism of the males in a species 

 of Leptoclielia. One form of male was distinguished l^y the great 

 development of the olfactory filaments on the antennules, and had 

 chelae very similar to those of the female. In the other form the 

 olfactory filaments were less numerous, but the chelae were greatly 

 elongated and slender. These observations have been doubted by 

 subsequent writers, but they have recently received partial con- 

 firmation from the work of G. Smith. 



Development. — The embryo shows at first a dorsal curvature as 

 in the Isopoda. A paired " dorsal organ " is present. The larvae 

 leave the brood-pouch with the last pair of thoracic limbs and the 

 pleopods undeveloped. In Apseudes, according to Claus, the lateral 

 plates of the carapace are at first extended as wing-like processes, 

 becoming afterwards bent downwards over the branchial epipodites 

 of the maxillipeds and fixed in position by peg-like outgrowths of 

 the sternum on each side. 



Eemarks on Habits, etc. 



The Tanaidacea are exclusively marine and occur from between 

 tide-marks to depths of over 2000 fathoms. Many burrow in 

 mud, some inhabit tubes of mud agglutinated by the secretion of 

 the dermal glands, and several species of Tanaidae are recorded as 

 living in rock-crevices among a felt-like mass of filaments, pre- 

 sumably also secreted by the animals. 



Most Tanaidacea are minute. Many species do not much 

 exceed one millimetre in length, but some Apseudidae reach 

 13 mm. or more. 



No fossil Tanaidacea are known. 



Affinities and Classification. 



Mention has been made above of the very varied opinions 

 which have been held regarding the systematic position of the 

 members of this order. Among recent writers, however, there is 

 general agreement that the Tanaidacea must either stand as a 

 distinct order or be merged in the Isopoda as the most primitive 

 sub-order of that group. While their resemblance to the Isopoda 

 in general form is considerable, their marked divergence in such 

 characters as the possession of a distinct carapace, with its branchial 

 chambers, and the form of the ejjipodites of the maxillipeds, fully 

 justifies their separation. 



