THE MAIN GROUPS OF ANIMALS. XXlX 



heat of the parents. In only one instance did the two series fail to 

 coincide, namely, in the case of the ovoviviparous fishes, which exception 

 is elsewhere explained. Omitting these, the series runs: (i) the fully 

 developed foetus of Vivipara, i.e. of Mammalia; (2) the perfect ovum of 

 Birds and Reptiles ; (3) the imperfect ovum of bony Fishes, of Cepha- 

 lopods, and of Crustacea; (4) the scolex of Insects ; (5) the generative 

 slime of certain Testacea ; (6) the bud, as in Bivalves, Ascidians, etc. ; 

 (7) the absence of all generative product and consequently spontaneous 

 generation, as in all lower groups ; and even occasionally in the higher, 

 up to Fishes. 



The Bloodless aaimals are dijwded by Aristotle »jto four great groups, 

 thq.'J'^stacea, theilssecta, the vjrustacea, and th^TCIqllusca ' (jidkaKia). 

 This last group corresponds to the modern Cephalopoda, and in the 

 following pages will be called by that name, to avoid the confusion into 

 which we should be led by retaining Aristotle's name Mollusca, this title 

 having acquired a different significance in modern times. At the bottom 

 stand the Testacea, in which are included all the modern Mollusca 

 excepting the Cephalopods, and also the Ascidians and the Echini. 

 Their inferiority to the rest is shown by their being completely or 

 almost completely incapable of locomotion ; by their having, that is, 

 no motor soul, or only dim traces of it ; so that they are repeatedly 

 spoken of as Sedentary (fiovifia), in opposition to all other true animals, 

 which are Locomotive (kivtjtiko.)' In this respect they resemble plants 

 rather than animals, as also they do in many other points, such as the 

 absence of sexual distinction,^ there being only one species, namely, the 

 Snails, in which there are any grounds for believing such distinction to 

 exist ; ' and in their mode of origin. For, like plants, they are generated 

 either spontaneously, or by budding, or from an excretion produced, like 

 the vegetable seed, asexually.* So nearly in fact do they resemble plants, 

 that they seem intended by Nature to occupy in the sea the place which 

 these occupy on the earth ; so that they may almost be spoken of as 

 sea-plants, while plants may be similarly looked on as land testacea.* 

 They graduate insensibly into plants through the Zoophytes ; the Echini 

 and Ascidians, which are usually reckoned by Aristotle as Testacea in 

 virtue of their external covering, being sometimes apparently included 

 by him in the group of Zoophytes. Still further signs of the inferiority 



' Aristotle having called these animals Mollusca, because their body consists of an 

 uniformly soft substance, tries also to define and name the three other groups according to 

 their consistence. The Ostracoderma [shell-skinned'), or Testacea, were soft within, and 

 hard without. The Malacostraca {soft- shelled), or Crustacea, were also soft within and 

 hard without ; but the hardness was that of a crusta, not of a testa. There remained the 

 Insecta. These, says Aristotle boldly, are uniformly hard throughout. But the statement 

 is so manifestly untenable, that he does not again insist upon it, nor attempt to give the 

 group a name expressing so false a notion. 



2 D. G. i. 23, 10. 3 D, Q, iii_ 11^ i8_ 4 D Q, iii_ 11^ II, 5 D. g. iii. 11, 5. 



