GENEBATION AND DEVELOPMENT. 199 



that they seemed to be produced from the so-called entrails 

 of the earth, apparently referring to certain worms formed 

 spontaneously in mud and the like.* 



The mode of reproduction of eels was in question for 

 many centuries, and although it had been known for some 

 years that there was a clear distinction of sexes among eels, 

 and that they passed from the rivers to the sea for breeding 

 purposes, it was not until about the year 1896 that the mode 

 of reproduction and development was fully ascertained. At 

 that time, Prof. Grassi and Dr. Calandruccio ascertained that 

 a fish, previously considered to be a distinct species, Lepto- 

 cephalus brevirostris, was a larval form of the common eel. 

 This larval form, which is flat and transparent and has a very 

 small head, passes through a series of metamorphoses into 

 the elver or young eel. The elvers swim up the rivers in 

 spring. Millions of them swim up the Severn, and have 

 long been believed by people in Gloucestershire to be young 

 eels. 



The egg-cases of whelks and other molluscs were known 

 to Aristotle,! but he was not disposed to believe that these 

 animals were generated otherwise than spontaneously. 



The metamorphoses of some of his Entoma received 

 much attention from Aristotle. According to him, all his 

 Entoma produce skoleJces,* or all, except certain Lepidoptera, 

 which produce seed-like bodies containing fluid. The 

 passages relating to his sJcoleJces are too numerous to be 

 given in full, but an epitome of the most important follows, 

 so far as the difficult nature of the subject permits. 



He appears to have been aware of the existence of the 

 ova or eggs of some of his Entoma, especially certain butter 

 flies and moths, locusts and spiders, || but considered them 

 to be, not eggs, but egg-like skolekes. The ova of many 

 Entoma escaped his notice, but he was aware of the existence 

 of their sJcolelces, and believed that these were the first 

 products of generation. IF The skolekes fed, grew rapidly, 

 and underwent changes, more or less complex, until they 

 passed into the pupa or chrysalis form.** 



The skolekes of the various kinds of Entoma are not 

 treated by Aristotle in the same way. When dealing with 

 those of bees, wasps, and the like, the larvae are called 



* H. A. vi. c. 15. 1 Ibid. v. c. 13. 



I O. A. ii. c. 1, 733a, H. A. v. c. 17, s. 1. 



j| Ibid. v. c. 17, SB. 1 and 4, v, c. 23; G. A. iii. c. 9, 15Sb. 



H G. A. ii. c. 1, 733&. * H. A. v. c. 17, ss. 4-6. 



