STRUCTURE AND PHYSTOLOGy OF THE MOLLUSCA. 49 



tion is nearly suspended, and injuries are not healed. They also 

 astivate, or fall into a summer sleep when the heat is great ; but 

 in this the animal functions are much less interrupted. {Midler.) 



Reproduction of lost parts. It appears from the experiments 

 of Spallauzani, that snails, whose ocular tentacles have been de- 

 stroyed, reproduce them completely in a few wrecks ; others have 

 repeated the trial with a like result. But there is some doubt 

 whether the renewal takes place if the brain of the animal be 

 removed as well as its horns. Madame Power has made similar 

 observations upon various marine snails, and has found that por- 

 tions of the foot, mantle, and tentacles, were renewed. Mr. 

 Hancock states that the species of eolis are apt to make a meal 

 off each other's brancJdce, and that, if confined in stale water, 

 they become sickly and lose those organs ; in both cases they are 

 quickly renewed under favourable circumstances. 



Reproduction hy gemmation. The social and compound tu- 

 nicaries resemble zoophytes, in the power they possess of bud- 

 ding out new individuals, and thus of multiplying their commu- 

 nities indefinitely, as the leaves on a tree. This gemmation takes 

 place only at particular points, so that the whole assemblages 

 are aggregated in characteristic patterns. The buds of the social 

 tunicaries are supported at first by their parents, those of the 

 compound families by the general circulation, until they are in a 

 state to contribute to the common weal. 



Viviparous reproduction. This happens in a few species of 

 gastropods, through the retention of the eggs in the oviduct, 

 until the young have attained a considerable growth. It also 

 appears to take place in the acephalans, because their eggs gene- 

 rally remain within some part of the shell of the parent until 

 hatched. 



Alternate generation. Amongst the tunicaries an example 

 is found of regulated diversity in the mode of reproduction. The 

 salpians produce long chains of embryos, which, unless broken 

 by accident, remain connected during life ; — each individual of 

 these compound specimens produces solitary young, often so un- 



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