of the Larvce of Marine Animals. 407 



bein^ continued to the seventli and eighth branchings, 128 

 and 256 heads, on the seventh and eighth successive nights. 

 Here we have an apparently ready means of determining the 

 age of the larvaj on which the Protozoa may be settled, but 

 one which fails absolutely, since I have, on some specimens 

 of the ascidian tadpole 20 hours old which Prof. Herdman 

 lias been kind enough to allow me to examine, found colonies 

 of such animals with 8 and 16 heads, 3 and -1 branchings. 



Crustacean Iarva3 may be soon disposed of. I have kept 

 zooeas unchanged for 12 days. I have found them and also 

 a few nauplii right to the west of Minikoi in the south-west 

 monsoon, not conceivably less than 25 to 30 days old, unless 

 there be shoals that we know not of or unless they belong to 

 adults which live below 1000 fathoms — an unlikely suppo- 

 sition. I have caught them on the east of Male in the 

 Maldives (lat. 4° 12' N.), and fancy from the currents that 

 they must have come rather from the Nicobars or the East 

 Indies than from Ceylon. We secured some to the south of 

 Funafuti, scarcely less than twenty days from the Phoenix 

 and Samoan groups, and my observations show that they 

 made nought of the journey of 250 to 300 miles from Fiji to 

 Kotuma, at least twelve days. Here is positive evidence 

 enough, and one is inclined to conclude that wherever any 

 bank may appear in the Indo-Pacific or Atlantic Oceans it 

 should be speedily provided with a fauna of such Crustacea as 

 possess free-swimming larvee of the zooean and more deve- 

 loped types *. 



The trochosphere is in its typical forms the larva of the 

 Polychffita, the Echiuroidea, and the MoUusca, modified but 

 yet quite distinct in the Echinoderms, Phoronis, and the 

 Enteropueusta, and also, though still more changed, of the 

 Nemerteans and the Sipnnculoidea. In its Echinoderm and 

 Enteropneustan forms it differs for different species almost as 

 much as do the adults, and so, perhaps, any direct research 

 on the subject may be expected to yield in these groups the 

 most definite results. According to Mortensen, in his 

 " Plankton-Expedition " report, these (Echinoderm) larvje 

 would appear to be seldom found in the high-sea plankton — 

 that of tlie open ocean. The inference would be that their 

 period of life is, under normal circumstances, only of a few 

 days' duration. As in the Sargasso Sea were found bipinnaria, 

 auricularia, and ophiopluteus larvaj at least 800 miles from 

 the nearest bank, and as it is in the highest degree improbable 



* I have not found it possible to distinguish between the niiuplius 

 larvfe of pelagic and littoral forms. 



20* 



