Classification of the Crustacea Malacostraca. 151 



in a thickening of the ectoderm on the sternal surface, and is 

 from the first completely closed. Further research will be 

 required to show what relation this cavity bears to the normal 

 marsupium. 



Apart from such exceptional cases, however, the possession 

 of oostegites is a character quite peculiar to the group of 

 orders included in the Peracarida and not found in any other 

 Crustacea. It is true that the Euphausiidse are described as 

 carrying their eggs in sacs attached to the sternal surface of 

 the thorax, and it has been assumed that these represent the 

 marsupium of the Mysidacea. But, as Sars * has pointed 

 out, the " ovisacs " are apparently formed by the consolidation 

 of some cementing substance which is extruded along with 

 the eggs from the oviducts. The rarity of ovigerous specimens 

 would suggest that the eggs are so carried for only a brief 

 period, while in some of the commonest species they have 

 never yet been observed. This last circumstance is explained 

 by an interesting observation for which I am indebted to 

 Mr. E. W. L. Holt. In Euphausia pellucida Mr. Holt 

 finds that the eggs when expelled from the bo ly are not 

 agglutinated together in masses, but are simply carried for a 

 time between the thoracic feet of the female. In Nyctiphanes 

 Coucliii the egg-sacs have long been known. By the kindness 

 of Mr. Holt I have been enabled to examine well-preserved 

 specimens of both these species, and I find that, as, indeed, is 

 implied by Sara's account, the structures found in N. Couchii 

 are more properly described as egg-masses than as sac?, there 

 being no definite encasing membrane, but simply a film of 

 hardened cement which also penetrates between and holds 

 the eggs together. It is plain that this structure bears no 

 morphological relation to the oostegites of the Peracarida. A 

 very similar arrangement is found in the Decapod Leucifer } 

 where, according to Brooks t> the eggs are " attached in a 

 loose bunch of twenty or more to the last pair of thoracic 

 limbs." 



A feature which is very characteristic of the Peracarida, 

 and one on which Boas and Hansen lay considerable stress, 

 is found in the structure of the mandibles. In all the orders 

 composing the series, with exceptions in the case of parasitic 

 and other secondarily modified forms, an accessory blade, the 

 lacinia mobilis of Hansen \ y is developed just behind the 



* Rep. Schizopoda ' Challenger,' p. 118. 



t " Leueifer, a Study in Morphology," Phil. Trans, clxxiii. (1882) p. 60. 



t The term lacinia mobilis was first applied by Hansen ('Dijmphna 

 Togtets Zool. Bot. Udbytte ' (1887), p. 197) to the accessory blade alone, 

 but he afterwards extended its meaning to include also the row of spines 



