190 THE ATLANTIC. [chap. iv. 



economy of the Ecliinoderms has been under the eye of a 

 greater number of observers, the development of the free-swim- 

 ming larva appeared to be so entirely the rule that it is usually 

 described as the normal habit of the class ; while, on the other 

 hand, direct development seemed to be most exceptional. I 

 was therefore greatly surprised to find that in the southern and 

 subarctic seas a large proportion of the Echinoderms of all or- 

 ders, with the exception, perhaps, of the crinoids (with regard 

 to which we have no observations), develop their young after a 

 fashion which precludes the possibility, while it nullifies the ob- 

 ject, of a pseudembryonic perambulator, and that in these high 

 southern latitudes the formation of such a locomotive zooid is 

 apparently the exception. 



This modification of the reproductive process consists in all 

 these cases, as it does likewise in those few instances in which 

 direct development has already been described, of a device by 

 which the young are reared within or upon the body of the 

 parent, and are retained in a kind of commensal connection 

 with her until they are sufficiently grown to fend for them- 

 selves. The receptacle, in cases where a special receptacle ex- 

 ists in which the young are reared, has been called a " marsu- 

 pium " (Sars), a term appropriately borrowed from the analogous 

 arrangement in their neighbors, the aplacental mammals of Aus- 

 tralia. The young do not appear to have in any case an or- 

 ganic connection with the parent ; the impregnated e^g from 

 the time of its reaching the morula stage is entirely free ; the 

 embryos are indebted to the mother for protection, and for nu- 

 trition only indirectly through the mucus exuded from the sur- 

 face of her perisom, and through the currents of freshly aerated 

 water containing organic matter brought to them or driven over 

 them by the action of her cilia. 



Animals hatching their eggs in this w^ay ought certainly to 

 give the best possible opportunities for studying the early stages 

 in the development of their young. Unfortunately, however, 



