ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 381 



ectodermal products and such of the mesodermal as are subjected to 

 the chitinizing activity of the ectoderm undergo ecdysis. The blood- 

 corpuscles, which, in Spiders, are formed at the expense of the endoderm, 

 are subjected to periodic modifications at every ecdysis, the result of 

 which is the proliferation of a large number of them. Besides these 

 there are other changes which occur principally about the time of 

 ecdysis, and are more or less closely connected with it ; they are by no 

 means confined to changes in size or to the complete development of the 

 genital organs. At the period of ecdysis Spiders are able to regenerate 

 organs which they have lost, and at that time they lose, more or less 

 early, certain provisional organs. 



6. Crustacea. 



Ancestral Development of Respiratory Organs of Decapodous 



Crustacea.* — Miss F. Buchanan attempts to answer the question how 

 the gills of Decapodous Crustaceans came to be situated in the different 

 positions in which they are found. 



The Chsetopod-like ancestor probably had no special respiratory 

 organs ; when the vascular surface became concentrated it would natu- 

 rally happen that the concentration would be in such parts of the body 

 as are most brought into contact with water ; and so, when certain limbs 

 became modified for swimming, the parts behind those limbs would first 

 become especially vascular and branchial. As it would be an advantage 

 to have the surface increased the surface became folded ; and a simple 

 respiratory plate is found in the nearest living representatives of the 

 Crustacean ancestor — the Phyllopoda. The typical thoracic appendage 

 of Apus consists of a basal or axial portion, six endites, and two exites, 

 viz. a flabellum and a bract. The fifth endite probably represents the 

 endopodite of the crayfish's limb, the sixth the exopodite, the flabellum 

 the epipodite, and the folded stem or bract is probably homologous 

 with the branchife of the Decapod. 



This primitive position of the respiratory behind the swimming 

 organ is retained in Schizopods, Stomapods, and in the higher group of 

 Isopods. The Archimalacostraca probably had not a settled respiratory 

 organ, for Nebalia has no special branchial organs. The next or archi- 

 schizopod form probably had a fixed number of segments, and differed 

 from the Archimalacostraca by its different manner of swimming, the 

 exopodite and not the epipodite being the branch used ; the respiratory 

 organs would, as usual, be developed on the swimming appendages. Of 

 living Schizopods, the Euphausiidae are the most nearly related to the 

 Archischizopoda, and they all have branchife attached to the bases of 

 their thoracic appendages, which are called podobranchise. In these 

 forms the gills have a very simple structure, for they merely consist of 

 branching lobes with no secondary branches. In the higher Lophogas- 

 tridse the gill is more complex, the primary lobes of the stem being 

 themselves lobed, and the attachment of the gill is not to the limb 

 itself, but to the arthrodial membrane near the base of the gill. In both 

 these groups the epipodites are reduced or absent on all the appendages 

 on which there are gills. 



In the Decapoda the thoracic feet have no longer a swimming func- 

 tion, and the exopodite has become vestigial or is altogether wanting ; 



* Quart. Journ. Micr. Sci., xxix. (1889) pp. 451-67 (1 pL). 



