.452 TRE GAT. [chap. xiii. 



Taking then the Lohder as an example of the ArUtropocla, we 

 find it to be a creature the body of whicli is different indeed from 

 that of a beast, bird or fish. Instead of an internal skeleton, we 

 find it has a skeleton whicli is external. Moreover the calcareous 

 portions of this external skeleton are moved (as so many levers, one 

 upon another) by muscles, as in the cat, but then these muscles do 

 not, as in the cat, clothe the skeleton externally, but are enclosed 

 within it. Instead of a Ijody the segmentation of which only 

 reveals itself internally (by the backbone and ribs), and which is but 

 slightly manifested externally (by at most two pairs of serially 

 homologous limbs), we find in the lobster a body consisting in great 

 part of externally visible segments, which it is plain are serially 

 homologous, and which are provided with numerous pairs of limbs, 

 which are also serially homologous one with another. Instead of a 

 pair of jaws biting vertically, and derived from special modifications 

 of the body- wall — i.e., from the visceral arches — we find a number 

 of jaws arranged in pairs and biting laterally, and which are really 

 limbs modified in a special manner. Instead of a frame made up 

 of two super-imposed cylinders, with the circulating centre ventral 

 and the nervous centre dorsal, we find a frame consisting of a single 

 cylinder, with the heart placed dorsally and the central part of the 

 nervous system extending from before backwards on the ventral 

 side of the single cylinder — while there is nothing answering to the 

 .cat's spinal column. Again, in the lobster the alimentary tube, 

 instead of bending away from the nervous centres at its anterior 

 end, not only bends towards them, but actually traverses them. 

 There are arteries and veins, but there is no portal circulation. The 

 organs of sense are also very different from the cat's. The lobster 

 has indeed a pair of eyes and also a pair of ears, but these are 

 situated on, or in, modified limbs and not in the head, and the eyes 

 have not various parts which exist in the cat, while the rods and 

 cones of the retina are placed at the surface and not at the deepest 

 layer of that membrane. In development, we have no such medul- 

 lary groove, becoming a tube, as in the cat (though the nervous 

 centres are still formed from epiblast,) nor do visceral clefts or 

 arches ever arise ; moreover it is the ventral, not the dorsal, surface 

 of the embryo which first appears. 



As the type of another sub-kingdom, that of the Mollttsca, we 

 may select the Cuttle-fi.sJt,, which exhibits a form as different from 

 the cat, as is that of the lobster, but also one as different from that 

 of the lobster as it is from that of the cat. For the cuttle-fish has 

 a large soft body, destitute both of the hard envelope of the lobster 

 and of the internal, axial and aj^pendicular skeleton of the cat, and 

 consisting of a fleshy bag, at one end of which is a head, with a 

 pair of large eyes and ten fleshy processes, or " tentacles," radiating 

 .from the terminal mouth of the bag. The body shows no serial 

 segmentation, cither externally, as does the lobster, or internally, as 

 does the cat. Neither does it show, like both these animals, an 

 elongated nervous axis, but instead, certain nervous ganglia (whence 



