44 NATURAL HISTORY OF ARTHROPODS. 



pouch beneath the thorax, from which the common name of these forms, " opossum 

 shi-imps," is derived. The floor of the pouch is formed from plates arising from the 

 bases of the legs in a manner similar to that found in the Edriophthalmia. When the 

 nauplius stage of the young is reached, the egg of Mysis hatches, but as a free life does 

 not begin until much later, the naupUus skin is not at once thrown off, and the subse- 

 quent changes are effected within it. 



Oedee III. — DECAPODA. 



This group embraces the largest, most interesting, and most useful forms of Crustacea, 

 and although the general structure is the same, the variation of the different parts in 

 size and proportion is such as to produce a great diversity in appearance. At first 

 sight the contrast between the shrimps, whose length is frequently ten times their 

 breadth, and Ixa, three times as broad as long, renders it difficult to realize that the 

 two are in any way related, though in reality every homology can be traced between 

 the two. Almost every part is essentially the same in each, the difference being only 

 in the "mode of expression," to use a term which belongs to the science of forty 

 years ago. 



The Decapoda, hke all of the Podophthalmia, have twenty segments of the body, 

 each, with the exception of the hinder one, bearing at some period of life a pair of 

 appendages. Of these, the two anterior (antennulsB and antenna) are especially devoted 

 to the senses ; next come six pau-s which play a part in eating, followed by five pairs 

 of feet (ten in all), which are of use in locomotion. On the abdomen are six pau-s of 

 small feet in the lower forms, while in the higher gi-oups, in the males, these are mostly 

 aborted, and in the females are iised only as supports for the eggs. As was stated on 

 a previous page, the terms head, thorax, and abdomen, when used in reference to the 

 Crustacea, imply functional and not morphological regions of the body ; and so, in treat- 

 ing of the Decapoda, the cephalic appendages extend from the antennulse to the ex- 

 ternal maxiUiped ; the thoracic members are the five large pairs used in locomotion, 

 while the abdominal legs embrace those on the seven last segments of the body. 



Going more into details, we will now discuss the various modifications of these 

 appendages and their functions. The anteriof pair, the antennulae, are always small, 

 and bear the ear on the basal joint. In some cases these antennulce terminate in a 

 single flagellum, while in others they are two, or even three-branched. The antennse 

 usually are much larger, and in the higher forms are unbranched, the exopodite dis- 

 appeai-ing with development. In the lower forms it, however, remains as a scale or 

 inconspicuous spine. On the basal joint is the external openmg of the " green gland," 

 an organ supposed to be renal or depuratory in function. Both the antenna and 

 antennulae are fringed with hairs, and are the special seat of feeling, and possibly of 

 smell as well. Of the mouth-parts the most anterior are the mandibles, a pah- of 

 powerful organs which play a part in the comminution of food, preparing it for 

 entrance into the mouth, which lies between them. The mandibles usually bear a 

 jointed continuation, the palpus, which in hfe assists in cleaning the cutting surfaces, — 

 a crustacean tooth-brush, it might be called. Two pairs of delicate, leaf-hke maxilla 

 come next, the first being without exopodite, while the second has this branch greatly 

 developed, forming the scaphognathite or gill bailer, to which reference was made on 

 a preceding page. In life this appendage is kept in constant motion, pumping water 

 over the gills. The next three pairs, maxillipeds by name, have the exopodite well 

 developed. 



