XXVIU THE MAIN GROUrS OF ANIMALS. 



those viviparous species, in which, as in the flesh-fly, the ovum is 

 hatched into a grub before being extruded from the parent's body ; 

 while the ordinary oviparous insects and spiders are those which produce ' 

 the motionless egg-like scolex. Why, however, it will be asked, should 

 Aristotle have considered the scolex, whether in shape of egg or of 

 grub, to be more immature than an ovum ? Simply because the con- 

 dition of scolex was antecedent to and preparatory of the condition of 

 chrysalis or pupa; and the chrysalis or pupa which neither ate nor 

 grew, and was the motionless form which immediately gave rise to the 

 perfect animal,' was supposed by him to correspond to the true ovum 

 of other animals.^ Though, however, it corresponded to an ovum, it 

 diff'ered from one because the whole of it was converted by metamor- 

 phosis into the perfect animal. Still less mature than the scolex was 

 the generative product of certain T^itacs^. This consisted at most of 

 a slimy fluid produced asexually, and scarcely diff"ering from the inorganic 

 mud, which could itself generate these animals spontaneously, though 

 its power of so doing was somewhat increased by the addition of this 

 excretion.' The slimy substances to which Aristotle alludes are the 

 agglutinated egg-masses of Gasteropodous molluscs. Although he failed 

 to see that these masses consist of a multitude of distinct ova, and are 

 not a simple homogeneous slime, yet in recognising them as the gene- 

 rative products of Testacea, he was in advance of the naturalists of the 

 eighteenth century, who described these egg-masses as distinct species 

 of animals, and gave them separate names.* 



The ova of other Testacea completely escaped Aristotle's observation ; 

 and he supposed these animals to be generated either spontaneously, 

 or by budding® from the parent; which is actually true of some of the 

 animals included by him in the group, but which he also supposed to 

 be true of the bivalved molluscs, such as Oysters and Mussels, which 

 live in large communities and are often found adhering to each other's 

 valves in masses, as though they had budded from each other.* 



There was thus a regular' series of gradations in the degree of maturity 

 reached by the generative product of diff"erent animals at the time of 

 birth, corresponding generally with similar gradations in the natural 



» D. G. iii. 9, 8, 9. 



* D. G. ii. I, 25. "The scolex after a time assumes the form of an epg ; for the so- 

 called chrysalis is equivalent to an egg." And again, iii. 9, 6. "The scolex as it becomes 

 matured and of larger size invariably at last assumes the condition of an ovum ; for 

 its outside hardens into a shell, and at this period it becomes motionless. This is readily 

 seen in the case of bees and wasps, and of caterpillars. The explanation lies herein ; that 

 the nature of these animals is imperfect, and that their eggs are consequently produced 

 before the due time, the scolex being as it were an ovum, still soft and in process of growth." 



' H. A. V. 15, 3 ; D. G. iii. 11, 12. etc. * Cf. M. Edwards, Le9ons s. 1. Phys. ix. 367. 



* D. G. iii. II, 12. 



•The spat of oysters was apparently first observed in the time of Pliny. "Nuper 

 compertum in ostreariis humorem iis fetificum lactis modo effluere." — N. H. ix. 74. 

 ' D. G. ii. I, 23. 



