458 Mr. E. Ray Lankester on the Planula- or 



It occurs just now in an advanced oviparous state, from which 

 the embryos are issuing. The latter are ciliated all round ex- 

 cept over the root-cells at the posterior extremity, like the 

 embryo of Halichondria simulans 5 but there is no ring of long 

 cilia round the base. It is also much smaller, measuring 22 

 by 15-1800ths inch in its greatest diameters, which brings it 

 near to the size of the embryo of Halisarca lobularis (PI. XX. 

 fig. 11) ; while it is remarkable for having the third form of 

 skeleton-spicule, viz. the acuate spiniferous one, together with 

 the two forms of flesh-spicules, alone developed, all of which, 

 as in other embryos, are . confined to the posterior end of the 

 body, where the former (that is, the spined acuates) lie grouped 

 parallel to each other, with their heads posteriorly and their 

 points anteriorly directed, not mixed up heterogeneously in 

 the cell-mass throughout the body (see the position in Hali- 

 chondria simulans, PL XXII. fig. 28, e). 



This sponge should come into my fourth division, or 

 AkmaT^E — that is, where the spicular skeleton-structure is 

 armed with spined acuates (the echinating spicule), as in 

 Dictyocylindrus and the like, since the third form of skeleton- 

 spicule above mentioned appears to be the latter. 



I must be pardoned for not believing in the existence of the 

 " bidentate " anchorate mentioned and figured by Dr. Bower- 

 bank here and elsewhere, which I believe to be an optical 

 illusion, since I have sought often and never been able to find 

 one. In no instance does an anchorate appear to me to 

 exist without the elements of the three arms or teeth at each 

 extremity, whether it be of the equi- or inequianchorate form. 



LVH. — Note on the Planula- or Gastrula-^ase of Development 

 in Mollusca. By E. Ray LankesterJ M.A. 



Prof. Salensky, of Kasan, in a recent paper in Leuckart and 

 Troschel's l Archiv fiir Naturgeschichte,' expresses doubts as to 

 the occurrence of a Planula- or Gastrula- phase of development 

 in certain Mollusks in which I have asserted its occurrence *. 

 I am anxious to make sonic reply to Prof. Salensky ; and, first 

 of all, I must ask him and others who, rightly enough, are not 

 prepared to accept " bare assertion " to wait until my drawings 

 are published in the ' Philosophical Transactions ' for 1875 

 before speculating as to whether I am right or not. Let me 

 repeat emphatically what is the condition I have observed in 

 the embryos of Pisidium, Tergipes, Polycera, Limax, Lymn&us, 

 and, I may now add, Paludina. The first cleavage of the 



* [A translation of Prof. Salensky 's paper has been prepared, and will 

 appear in the next number of the ' Annals.' — Ed.] 



