History of British Entomostraca. 405 



all the previous writers. Jurine describes six species which had 

 not been described by Muller, and Straus describes three, though 

 some of them seem to be only varieties. Desmarest, in his work 

 " Consid. Generales sur les Crustaces," 1825, enumerates fourteen 

 species which had been described by the authors who had written 

 before his time, and which have been found in France — but adds no 

 new ones. Gruithuisen has published a very interesting memoir 

 upon the D. sima (Muller) in the 14th vol. of the " Nova acta Phy- 

 sico-Medica Academia Csesariae Naturie Curiosorum/' part 1st, 1815, 

 in which he describes at some length the circulation of the blood as 

 observed by him in this insect. He describes two hearts, the arte- 

 rial heart and the venous heart, and gives a figure much magnified 

 of the blood in motion. * His figure of the insect itself, however, 

 (Fig. 1-2,) is either not very correct, or it is a species different from 

 the Sima of Muller. 



Habits and Manners. — These insects are only to be found in 

 fresh water, generally in ponds and ditches, where there is much 

 of the lemna or duck-weed floating on the surface. In such places 

 they are often to be found in myriads, and almost the whole year 

 round ; and as they sometimes, and in some species, assume a 

 reddish colour, they have been said to have tinged the watei 

 with the hue of blood. Swammerdam asserts this to be the case, 

 and says that he has seen them in such numbers at Vincennes, as 

 actually to give the water of a horse pond the colour of blood; 

 and he quotes a friend of his in Holland, a Dr Schluyl, who had 

 observed the same occur in one of the canals near his house. This 

 has been repeated by Derham, in his Physico-Theology,t and by 

 others upon Swammerdam's authority ; but no writer since Swam- 

 merdam's time has observed it himself. They are very prolific, hav- 

 ing a great many layings, and some of the larger species having been 

 observed to have as many as forty or fifty eggs and upwards in their 

 matrix at once. According to Jurine, in June the young ones be- 

 gin to have eggs about ten days after birth, and it is probable they 

 continue to produce all the summer through at frequent intervals. 

 The males are very few in number compared with the females, and 

 are only to be met with at certain seasons. From this circumstance 

 SchoefFer and others have considered them as Hermaphrodites ; and 

 Sulzer, (as quoted by Straus,) though he oppugns this, gives a more 

 singular opinion still, being of opinion that a copulation might take 

 place with the young before they see the light of day ! These au- 



» Tab. xxiv. Fig. 6. f P. 364— ( Note a. ) Glasgow, 11th edit. 1745. 



