246 INSECT TRANSFORMATION 



The trochophore in elaboration of structure is distinctly 

 in advance of the planula or gastrula, but it is in a very 

 early stage of development as compared with the adult 

 into which it has to grow. It is further noteworthy that most 

 annelids and molluscs that live on land earthworms and 

 snails, for example are hatched in an advanced stage of 

 development, displaying already the distinctive structural 

 features of their parents ; a minute and delicately-built larva 

 is altogether unfitted for terrestrial surroundings. Also it is 

 well to remember, in making these comparisons as to develop- 

 mental stages in different groups, that with some marine 

 molluscs, the cuttle-fishes and their allies, the young are not 

 turned out to begin their free life as immature larvae, but are 

 hatched with all the essential features of the adult ; these 

 Cephalopoda have much larger eggs than most Mollusca have, 

 with a rich supply of yolk for the nourishment of the embryo, 

 so that provision is made for its building-up to a high degree 

 of development. 



It may now be instructive to turn to various types of growth 

 and transformation that are exhibited by members of various 

 classes of the Arthropoda, the great group of animals in which, 

 as we have seen (pp. 5-6), insects are included. All arthropods 

 produce relatively large eggs with a considerable accumulation 

 of yolk, but they show striking differences in different orders 

 and families as to the degree of change undergone after 

 hatching. If, for example, we take the Crustacea, that class 

 of Arthropoda which are typically aquatic and mostly marine 

 in their manner of life, we find that in the assemblage of 

 orders known as the Entomostraca comprising the water- 

 fleas, barnacles, and their allies, on the whole low-grade types 

 of Crustacea the young is hatched as a naiipliiis larva, 

 (Fig. 120 a] a form in some respects highly organized, and yet 

 incompletely grown, having only the three foremost pairs of 

 appendages, the hinder series of segments and their appendages 

 being afterwards developed successively from before backwards 

 through a series of moults. Among the higher Crustacea, on 

 the other hand, the nauplius larva is very rarely found ; either 

 the young is hatched with all its segments and its thoracic 

 appendages, growing the abdominal limbs through a series of 

 moults, with transformation in points of detail, as in the life- 



