134 
breeding season and the occurrence of some species in the locality, 
where the researches were carried out. Thus, when there are found 
in this place four different species of Spatangoids, the development 
and the larvæ of two of them being known* the third having its 
breeding season at another time of the year, while the fourth has 
its breeding season just at the time in question, it is a logical con- 
clusion that when a larva not belonging to any of the two known 
forms appears in the plankton, it must belong to the fourth of 
those species. Brissopsis lyrifera represents such a case. In the 
same way, if some larva of say an Ophiuroid is found to occur 
in considerable numbers in a place, where only one species of 
Ophiuroid occurs, it is no less justified to conclude that this larva 
must then belong to that species of Ophiuroid. Such a case is 
represented by Ophiura texturata in the Limfjord; another quite 
similar case is that of Echinocardium cordatum. MacBride in 
his memoir on the development of Echinocardium cordatum ! ), 
objects to the latter case, stating that the identification of my spec- 
imens as the larvæ of that species „rests on a mere guess because 
the ground of the identification was the occurrence of the larva in 
question in the waters of the Limfjord in Denmark on the sandy 
bottom of which Echinocardium cordatum was the only Spatang- 
oid which was found“. I must protest against designating that 
determination as a „mere guess“; Echinocardium cordatum being 
the only Spatangoid occurring in the Limfjord 2 ), it is not mere 
guess to refer a Spatangoid larva normally occuring there to that 
species. It is a logical conclusion, the correctness of which Mac¬ 
Bride could only ascertain. Mere guess work it would be, if I 
had referred such forms as Ophiopluteus ramosus or O. coronatus 
to some definite Ophiuroid, for which reference there would be 
1 E. W. M a c B r i d e. The development of Echinocardium cordatum. Part. I. 
The external features of the development. Quart. Journ. Mier. Sc. Vol.59. 
1914. p. 472 
5 Since MacBride (op. cit.) mentions especially the sandy bottom as 
the condition under which Echinocardium occurs in the Limfjord, this 
might possibly indicate the meaning that other Spatangoids were to be 
found in places with muddy bottom in the same locality. But, anyhow, 
there is no foundation for such supposition; as I have stated expressly 
(Echinodermenlarven d. Plankton-Expedition, p. 103), Echinocardium cor¬ 
datum is the only species of Spatangoids occurring in the Limfjord. 
