ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 435 



Its early existence appears to be precarious. Deposits of young shells of 

 fifty to the square foot, after five or six months were found almost com- 

 pletely devoured. A peculiarly insidious enemy is a small whelk, which 

 pierces a neat hole in the thin shell, generally in the neighbourhood of 

 the attachment of the adductor muscle. The result -is loss of power to 

 close the shell against the enemy, and every returning tide washes up 

 numbers of young shells perforated in this way. The oyster does not 

 appear to be altogether passive as regards its enemies, for where the 

 situation is an exposed one the young form makes a tour in search of a 

 cranny where it can hide. They often, however, only survive a few 

 months, the secluded spots selected proving too small for their growing 

 shells. For the purposes of locomotion a modified foot, like a muscular 

 thread, is protruded from the byssal cleft. It gropes and tests the 

 ground in every direction, eventually fixing itself and drawing the oyster 

 after it. The latter then rests on its flatter, right side. Power of in- 

 dependent movement is confined to shells of six months age or under. 

 The foot atrophies during the latter part of the bivalve's existence. On 

 the other hand, the formation of new byssi is possible, and, if occasion 

 demands it, occurs in shells of almost any age. The author makes an 

 interesting comparison between the present species, M. margaritifera, 

 and M. maxima. These three show grades of modification in hinge- 

 teeth characters (rudimentary or absent), in nature of byssus, and in 

 presence or absence of a modified foot. 



Arthropoda. 



a. Insecta. 



Polyembryony and Sex-Determination.* — E. Bugnion calls atten- 

 tion to the fact that in Encyrtus fuscicoUis all the individuals hatched 

 from one parasitised caterpillar are usually of the same sex. P. Marchal 

 observed the same in the individuals of Polygnotus minutus which issue 

 from one parasitised Cecidomyid larva. The fact is a natural consequence 

 of the polyembryony. As in the case of identical twins, the sexes of 

 the products of one ovum are the same. When one parasitised cater- 

 pillar includes two or three ova of Encyrtus, and therefore several chains 

 of embryos, both males and females may be hatched out. The partial 

 abortion of one chain may result in unequal numbers of both sexes. As 

 Bugnion points out, the facts seem to show that in this case the sex is 

 pre-determined in ovo. 



Assimilation of Carbon Dioxide by Chrysalids.| — Maria von 

 Linden has experimented with chrysalids of Papilio podalirius, Sphinx 

 euphorbia and Lasiocampa pini, and with caterpillars of Botys urticata 

 and Vanessa urticm, and has been led to conclude that when the atmo- 

 sphere surrounding them contained 5-30 p.c. of carbon dioxide, there 

 was often an absorption of that gas, accompanied (in spring) by an 

 exhalation of oxygen. This process of assimilation occurs oftener 

 during the day than at night, but respiration is more intense during the 



* Arch. Sci. Phys. Nat., xx. (1905) pp. 699-702. 

 f Comptes Rendus, cxli. (1905) pp. 1258-60. 



