PREFACE. xi 



oval ones, deeply stained by haemoglobin. The number of these corpuscles 

 is so considerable as to give the blood of Solen legumen a bright blood- 

 red colour. 



I may add here that I have observed similar though larger corpuscles 

 impregnated with haemoglobin in the blood of species of Area. 



Homologies of the Arms of the Cephalopoda. — The view that 

 the sucker-bearing arms of the cuttlefish are to be regarded as appendages 

 of the head homologous with the tentacles on the head of Gasteropods 

 (p. 326), is one which, it will be well for the student to remember, is not 

 that usually taught. He should make himself acquainted with the older 

 and the neAver view, and the grounds on which they are based. Without 

 entering into a discussion of the arguments which may be adduced in favour 

 of this or of rival interpretations of the parts, it must suffice here briefly to 

 mention that the arms of the Cephalopod (the development of which had 

 been made known by Kolliker), were shown by Professor Huxley, 

 five-and-twenty years ago, to correspond to the fore-part of the foot of 

 the Gasteropoda, and the ganglion, from which they receive their nerve 

 supply, was then considered as corresponding to the pedal (Morphology of 

 the Cephalous Mollusca, Phil. Trans. 1853). This view was maintained in 

 the earlier editions of Gegenbaur's work. It has been abandoned in the 

 present edition, in deference to the statements of Mr. Jhering (" Vergleichende 

 Anatomie des JSTervensystems und Phylogenie der Molluscen, Leipzig," 1877). 

 The whole of that author's work, both statement of fact and speculative 

 superstructure, appears to me to call for very cautious treatment, involving 

 the rejection of some of his principal conclusions. 



Origin of the Limbs of Vertebrates. — Professor Gegenbaur is 

 inclined to regard the skeleton of the limbs and limb-girdles of Vertebrata as 

 derived from gill-arches and their branchial rays (§ 357). The student is 

 reminded that another possible derivation of these organs is from primitively 

 continuous lateral fins — supported by cartilaginous rays, and comparable to 

 the primitively continuous dorsal median fin. The specialisation and con- 

 centration of the lateral fin on each side in two regions, thoracic and pelvic, 

 woidd be competent to give rise to the two pairs of fins, such as we find in 

 the Elasmobranchs. Mr. Balfour (" Development of Elasmobranch Fishes," 

 1878) is led to adopt this view by the observation, that in the embryo dog- 

 fish the lateral fins have precisely the same mode of origin as has the 

 dorsal median fin, arising " as special developments of a continuous ridge 

 on each side, precisely like the ridges of epiblast, which form the rudiments 

 of the unpaired fins." This view of the nature of the vertebrate limbs has 

 been independently worked out with great care from the point of view of 

 comparative anatomy, by Mr. J. K. Thacher (Median and Paired Fins, 

 Transactions Connecticut Academy, vol. iii. 1877). In the important 

 memoir just cited, Mr. Thacher shows very plausibly how the Elasmobranch 

 fin, and not only the fin, but the supporting limb-girdle also, may have 



