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Material for Evidence.
The evidence upon which this investigation is based is as follows:—
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(a) Our previous knowledge of the indirect, pelagic type of development. This is founded mainly on the researches of MacBride (1907) on Ophiothrix fragilis and the recent work of Narasimhamurti (1933) on Ophiocomina nigra.
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(b) Our previous knowledge of the development of a moderately yolky ophiuroid egg. This is founded solely on Grave's work on Ophiura, brevispina (1900 and 1916).
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(c) The recently published work of the writer on Kirk's ophiuroid, a form with very yolky eggs, and on Ophiomyxa brevirima, a viviparous form also with very yolky eggs (1940 a, 1941).
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(d) The scattered body of data in the literature in regard to isolated stages of development of other ophiuroids—mainly referring to larval forms.
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(e) The research presented elsewhere (Fell, 1945) on Amphipholis squamata, a form with moderately yolky eggs and of viviparous habit.
It will be seen from the above list that there is a fairly representative body of data available, ranging from non-yolky eggs to those with a very large proportion of yolk, and including cases of viviparity. In addition to these data, there is a considerable amount of scattered information on the other echinoderm classes, but in no ease presenting so complete a sequence as is now available from the Ophiuroidea.
The method adopted in this investigation is comparative and analytical—i.e., each of the principle phases of development is taken separately, and examined with reference to the forms showing indirect development, those showing intermediate characters, and those showing direct development.
What is the “Typical” Echinoderm Development?
Before commencing the study of the nature of direct development in echinoderms, it will be useful to examine the position in regard to development in general in this group. As pointed out in the introductory section of this paper, it has come to be generally assumed that all “typical” echinoderms have an indirect development; conseqently, whenever an echinoderm has been found which departs from this criterion, it has been glossed over in the text-
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books with a remark to the effect that it is “not typical”—and hence unworthy of further consideration.
Examples of this attitude are abundant. Thus, commenting on the development of Antedon, MacBride (1914) states, “We may discount the characteristic features of the development of Antedon, which are obviously due to the yolky egg …”, etc. Antedon, it should be noted, is the only crinoid of which we have any embryological knowledge, yet, because it lacks a larva comparable to the Ophiopluteus or Bipinnaria it is automatically rejected as “atypical.” for, it is stated, a “typical” crinoid must undoubtedly possess a bilaterally symmetrical and non-yolky larva.
If echinoderms with yolky eggs—and consequently having direct development to a greater or less degree—are held to be atypical, then obviously a census of the echinoderms of whose development we have any evidence should reveal at once that directly developing forms constitute the great minority. It should be found that most echinoderms have an indirect development, coupled with a pronounced larval stage. Is this the true position? In order to obtain some information on this question I have selected three specimen faunas for special examination. These are the echinoderm faunas of Great Britain, of New Zealand, and of the Antarctic and Sub-Antarctic. In these we may expect to find reasonably typical samples of the world's echinoderms.