X PEEFACE. 



by the fact, that in Sigahon the elytra and dorsal cirri exist side hy side on 

 the same segment. 



Homologies of the Eami of the Appendages in Astacus. — The 

 view taken by Professor Gegexbaur, as to the homologies of the parts of the 

 appendages immediately following the mouth in Astacus, differs somewhat 

 from that Avhich is current in this country. In Fig. 122, p. 239, the 

 mandible, two maxillae, and three maxillipedes of the right side of Astacus 

 fluviatilis are figured. This woodcut was kindly re-drawn for the English 

 edition by the author, at my request, and gives a more complete outline of 

 the parts in question, than does the older cut of the German edition. 

 Throughout the series of appendages, tluee divisions are distinguished by 

 the letters a, e, d. Taking the lowest figure first (the third niaxillipede) we 

 find the endopodite marked a, the exopodite marked c, and tlie letter d 

 placed with the single epipodite (podobranchia, Huxley) to its inner side, 

 whilst the double arthrobranchia (Huxlet) not forming part of the appendage 

 proper, but a distinct respiratory development, is seen on its outer side. In 

 the next figure (the second niaxillipede), a indicates endopodite, c exopodite, 

 and d is placed close to the double arthrobranchia on its outer side, whilst 

 the modified epipodite is seen to the inner side again, of this. In the figure 

 of the first maxUlipede, a is placed near the foliaceous endopodite, which 

 has a detached outstanding segment, e near the filamentous exopodite, and 

 d near the broad epipodite. The same exi^lanation of the lettering holds 

 good for the next appendage, the second maxilla. In the next appendage — 

 the first maxilla — the absence of the letters c and d, indicates that the 

 author regards the whole appendage as reduced to the representative of 

 the foliaceous endopodite a of the two inferior appendages — a view with 

 which few will disagree. In the case of the mandible, hoAvever, Professor 

 Gegexbaur marks the " palp " with the letter c — considering, therefore, the 

 basal piece of this appendage to represent the endopodite, and the palp to 

 represent the exopodite. The more usual opinion on this matter is that the 

 mandible, together with its palp, corresponds to the simple foliaceous first 

 maxilla. The jointed palp, mounted on its solid basal biting-piece, cor- 

 responds to the jointed endopodite a of the last maxillipede. 



The question of the presence or the absence of a representative of the 

 exopodite in the Decapod's mandible, is a matter of considerable importance 

 in reference to possible comparisons between the gnathites of Crustacea and 

 Tracheata. The actual development of the parts in question from the 

 nauplius-form of appendage, must be the ultimate test of the homologies of 

 their rami in the Crustacea. 



Blood-corpuscles of the Molluscn. — The statement on p. 375, that 

 " the form-elements of the blood are always colourless " in the IMollusca, is 

 one which I may be allowed to correct, since I have published an account of 

 the blood-corpuscles of Solen legumen (Proc. Eoyal Society, I^o. 140, 

 1873), Avhich^ besides colourless amoeboid forms, comprise a vast number of 



