CHAP, xm.] TBE CAT'S PLAGE IN NATURE. 453 



nerves radiate,) aggregated near the moutli and surrounding the 

 alimentary tube. It has eyes, indeed, which are very like the cat's, 

 yet the rods and cones of the retina are placed as in the lobster. 

 There are no limbs laterally arranged in pairs, as in the cat and 

 lobster ; and though it has a pair of jaws, they are no more modi- 

 fied limbs — like the lobster's — than they are modified visceral arches 

 ■ — like the cat's ; for such arches are not formed in its development, 

 any more than are a medullary tube and a notochord. 



The lobster and cuttle-fish are examples of the most highly- 

 organized sub-kingdoms which make up the Invertebrata, but some 

 species of a sub-kingdom which is in most respects inferior, yet 

 present certain noteworthy approximations to cat-structure. The 

 sub-kingdom referred to is that of the Tanicata. Some of these 

 Ascidians are marine animals, with a leathery coat, or "test," shaped 

 somewhat like a bottle with two short necks. Such a creature is 

 alike devoid of anything, either in the shape of an external limb or of 

 an internal skeleton. Its nervous system consists of little more than a 

 ganglion, while (as in a multitude of Invertebrata of difi'erent kinds, 

 both male and female sexual glands co-exist in each individual. 

 Such is its adult condition. Yet during its growth it possesses a 

 medullary groove that becomes a dorsal tube developing the nervous 

 centres, which are temporarily in the form of an elongated axis ; 

 while beneath it a structure is formed which closely resembles the 

 chorda dorsalis of the cat's embryo. It is only in some Tunicata, 

 however, that this condition temporarily obtains, though in all, the 

 nervous centres are dorsal in position and origin. There are no 

 visceral clefts and arches as in the embryo cat, yet a number of 

 serial openings are formed on each side of the pharynx, which 

 persist throughout life, and through which water is propelled by 

 the lashings of vibratile cilia. 



All the creatures yet mentioned resemble the cat, inasmuch as 

 they exhibit either a serial or a bilateral symmetry, or both. 



But there may be another form of symmetry (entirely absent in 

 the cat,) namely, a radial synnnetri/, which is exhibited by jelly- 

 fishes, sea-urchins, star-fishes and others. 



In various Invertebrate sub-kingdoms, various forms of organic 

 inferiority are found. Some animals grow up in compound 

 aggregations, as do so many of the Coilentera. These last-named 

 animals have a certain interest for the student of Cat embryology. 

 We saw that in the fertilized ovum of the cat, yelk division ended 

 in the formation of two layers — one superficial layer of epiblast, 

 with a hypoblastic layer beneath it. Amongst the Coelentera we 

 find animals whose whole body, through their whole lives, consists 

 only of two corresponding layers : an external layer, the " ecto- 

 derm," and an internal layer, the "endoderm;" the endoderm 

 (like the embryonic hypoblast of the cat,) lining the alimentary 

 cavity. These resemblances are not merely fanciful, for the condi- 

 tions, which exist in a multitude of intermediate forms, show that 

 there is a real affinity of nature between those corresponding parts 



