OF THE SALP.E. 49 



the liatcliing of the young fry, Müne Edwards observed 

 theh- development at periods of four hours, and has given 

 figiu'es for each of these periods which represent the 

 changes undergone by the animal in the intervals. 



The young animals, as it was correctly stated by Sars, 

 present externally a delicate, lax integument, hanging- 

 round them in sacculated folds in consequence of the 

 retraction from it of the soft internal parts ; which are 

 otherwise of the same shape as the external sacciform 

 case, only that there proceed from them anteriorly several 

 cylindrical prolongations, dilated at the end into a sort 

 of knob, by which they are in contact with the external 

 tegument, to which they are the more closely united, the 

 shorter the time since the escape of the embryo from the 

 egg. These organs, thus placed with considerable regu- 

 larity within the integument and around the anterior ex- 

 tremity, misled Sars into the supposition that it was not 

 a single embryo which was contained in the egg, but a 

 whole circle or star-shaped assemblage of them. Milne 

 Edwards, however, shows that these dilated extremities 

 serve as suctorial discs, which are employed by the 

 animal after it has swam about for some horn's in the 

 water, to attach itself by to larger objects, in order to 

 become fixed like the parent animal. At this period a 

 change in the development begins to occur. The pro- 

 cesses, which support the suctorial discs, and the in- 

 terior caudal tube become absorbed, and the substance 

 of which they were composed gradually enters into that 

 composing the bulk of the soft parts, that it may be 

 formed into new organs ; and in about ten hours from the 

 time of its affixing itself, the animal is contracted into a 

 globular mass within the wide, dead, caudate integument. 

 In twenty hours, the situations of the future mouth and 

 heart become evident ; on the next day the oral orifice is 

 completed, and water begins to enter the respiratory sac; 

 on the third day the pulsation of the heart is observable, 

 and the outward form of an Ascidia recognizable, and 

 weithin a few days all the organs are formed, except those 



4 



