92 FRESHWATER ANIMALS 



paper, the differences in the relative growth of the exoskeleton 

 of these body parts are especially noticeable because they alter the 

 outline of the body when the animal is viewed laterally, as it 

 usually is. When the extreme forms (A and C) are viewed in the 

 dorsal aspect, the chief difference is in the extension of a very 

 thin lamella which develops over the midline of the head and 

 carapace. It is this thin lamella, composed of two closely op- 

 pressed sheets of exoskeleton separated by a thin film of haemo- 

 lymph, which is responsible for the most striking shape differ- 

 ences between the variant individuals. The mid-dorsal extension 

 of the head margin is called a "crest" when it covers a relatively 

 broad section of the anterior and dorsal portions of the head 

 margin, and a "helmet" (see Figs. 3, 4) if narrower at the base 

 and taller. The bases of the antennule muscles are included in 

 the drawings to indicate the position of the body margin (in 

 lateral view) if the crest or helmet were not present. As might 

 be expected, the relatively excessive exoskeletal growth of B and 

 C as compared with A is also expressed in the larger and more 

 numerous spinules along the dorsal line of the carapace and on 

 the ventral margins of the valves, and in the relatively greater 

 length of the shell spine. Although in the species depicted here 

 the most exuberant form ( C ) is larger than the most conservative 

 (A), this is usually not true; the individuals with the most exag- 

 gerated shapes often are smaller. It is therefore not a question of 

 increasing disproportion with increasing size. 



This wide range of phenotypic variation occurs in generations 

 that have developed parthenogenetically. Various laboratory ex- 

 periments on Daphnia (see especially Brooks, 1946, 1948) indi- 

 cate that a clone formed from any individual picked at random 

 from a natural population can express the same range of pheno- 

 typic variation that the natural population exhibits under similar 

 environmental conditions. Although various genetic mechanisms 

 are known in diverse organisms by which recombinations are pos- 

 sible even when the orthodox meiotic process is absent, there is 

 no evidence that they play a role in the determination of the 

 phenotypic variation in Daphnia. Tin's does not mean to imply 

 that some such mechanism may not be effective in increasing 



