6574 Crustacea. 



type, but passes not through all the inferior forms. The higher forms, 

 such as the short-tailed crabs, skip over those of the stomapod type, 

 and put on more early their own peculiar features, whereas the young 

 of the Mact'oura, or long-tailed genera, enter more decidedly into the 

 forms that are beneath them ; and at a certain stage the larva of the 

 prawn and shrimp can scarcely be detected from that of the adult 

 opossum shrimp. Great outcries are made against those philosophers 

 who endeavour to support the doctrines of the unity of animal crea- 

 tion ; but there appears to us nothing outrageous in the idea that 

 supposes the adult form which resembles the intermediate stage in the 

 development of one of a higher type to have been the result of 

 an arrest in the progress of that particular species. We know that 

 an arrest in growth may take place under certain conditions for an 

 indefinite length of time, and that during that period they even pro- 

 pogate their kind, but when those conditions are removed animals pro- 

 ceed to a higher degree of development, and complete the history of 

 their lives. This has been shown to be true of the Tenoid 

 worms, which remain unchanged and reproduce their own kind while 

 inhabitants of one animal will, upon being devoured by another, pass 

 on to the adult condition. If this be true for a period, we have only 

 to suppose the conditions fixed, to make the arrest permanent, when 

 it must take its place as a separate species among the rank of 

 animals. 



In the development of the common shrimp, the progress of the 

 animal is so nearly through the form of Mysis, that an arrest of 

 development of the animal at that particular stage would place it in 

 the genus. Now, whether the power to change the conditions, more 

 or less favourable to the animal, or the actual creation of a new crea- 

 ture is the more consistent in the great plan of creation, there is at 

 present not sufficient evidence for us to come to correct conclusions. 



We know that altered circumstances induce a variety in form and 

 size. The Crustacea that are large and spinous in the Arctic regions 

 dwindle in dimensions and become less spinous in the temperate zones : 

 these changes made constant appear the only separation between the 

 variety and the species. 



When the crab first becomes a walking animal, it bears no very 

 distant resemblance to one of the triangular species', narrow in the 

 front and broad towards the posterior extremity of the carapax. The 

 spider-crabs, as they are nicknamed, from a general resemblance they 

 are supposed to bear to a spider, have been placed as the highest 

 forms in this class, but observation demonstrates that in the course of 



