ON LARV.E OF LINGULA AND PELAGODISCUS (DISCINISCA). 59 



apparently simple, i.e. not spinulose, and a series of finer chaetae on the lateral 

 margin of the mantle. Eichler and Blochmann consider that these larvae probably 

 belong to Pelagodiscus (Discinisca) atlanticus ; but there is no justification for 

 referring them to this species, from which they differ in the following characters : 

 their much greater size, the form of the median tentacle, their cirri (which are 

 about three to four times as long as those of P. atlanticus), and in their possession 

 of only four principal chaetae, the longest of which is not specially stout. These 

 larvae differ so strikingly from those already referred on good grounds to 

 P. atlanticus that they cannot be regarded as belonging to the same species ; they 

 may be larvae of some larger species of Discinisca. 



The Larv/E of Pelagodiscus (Discinisca) atlanticus. 



Six free-swimming larvae were taken on October 16, 1914, in the Indian Ocean a 

 few miles west of Cape Comorin, where the chart shows depths of about 40 fathoms. 



Adult specimens of P. atlanticus have been found almost entirely in deep water ; 

 there are records from 200 and 690 fathoms, but with these exceptions specimens 

 have been found only at depths greater than 1000 fathoms, and there are four 

 records from more than 2000 fathoms, the deepest being 2425 fathoms. The 

 occurrence of the larvae of Pelagodiscus off Cape Comorin in shallow water was 

 therefore rather unexpected. The fact that half a dozen larvae were obtained 

 together indicates the probable close proximity of the parent forms. There is, 

 however, the possibility that the adults lived in the deep water to the west, for 

 the depth increases rapidly in that direction, and there was deep water — 700-800 

 fathoms— only about thirty miles away. It is known that the larvae may remain 

 free-swimming for five or six days,* a period which would have been sufficient for 

 their transportation by the strong currents from the area of deep water to the 

 locality where they were found. Although the conditions suggest that the adults 

 of P. atlanticus occur in shallow water near Cape Comorin, a definite conclusion 

 cannot be reached on the evidence available. 



I have recently looked over charts f of the other areas from which larvae of 

 Pelagodiscus have been recorded. Around Desterro, where Muller found his 

 larvae, the sea is shallow, the nearest water of 100 fathoms depth being some 

 sixty miles eastwards. Off Bin tang, where Professor Blochmann's ten larvae were 

 taken, the water is very shallow (0-24 fathoms), the nearest water of 100 fathoms 

 depth being about four hundred miles away. At Misaki, where Dr Yatsu took a 

 single larva, there is deep water close inshore. It is evident from a consideration 



* Muller (1861, p. 54) observes, regarding hislarvse in aquaria: "Die Dauer dieses Schwarmstadiums iiberstieg 

 bei den eingefangenen Larven nie 5-6 Tage, meist schon friiher setzten sie sich fest, am Boden oder an den Seiten 

 des Glases." Schuchert (Bull. Geol. Soc. Amer., vol. xxii, 1911, p. 272), however, states that the larvae " are known 

 to live in the free and floating condition for nearly a month," but there is no evidence in support of this statement, 

 which is erroneous. 



t I am indebted to Dr W. S. Bruce for giving me access to the charts in the Scottish Oceanographical Laboratory. 



