366 DR F. A. BATHER. 



and since no " Eustelea " come up for description in this memoir, we may now turn 

 back to the Heterostelea. 



§ 20. The chief advance that is fixed in the conception of a taxonomic group 

 Heterostelea is the union of the Dendrocystidae with the Anomalocystidae, mainly on 

 the oTOund of their biserial columnals. If the Eustelea be removed, then the Hetero- 

 stelea coincide with the Carpoidea, aud the whole conception becomes more distinct. 

 In its various negative characters the assemblage still comes under the Amphoridea as 

 defined above, but may be separated as a Suborder, for which the name Heterostelea 

 may still be employed. The diagnosis will then run as follows : — 



Suborder : HETEROSTELEA. 



§ 21. Diagnosis. — Amphoridea in which there are two planes of symmetry inter- 

 secting one another at right angles along the main axis of the animal : about the one 

 (the extension-plane) the theca is compressed, and with reference to this plane tends 

 to difference of structure ; with reference to the other (the symmetry-plane) both theca 

 and stem tend to bilateral symmetry of form, which, in the case of the stem, results in 

 the diraerism of most or all of its columnals. A stem is always present, though not 

 necessarily used for fixation in adult life. 



§ 22. Terminology. — In this diagnosis the attempt has been made to describe the 

 differentiating modifications in an objective manner, in so far as objectivity is consistent 

 with any assumption that they are modifications of a pre-existing form. The diagnosis 

 would be far more intelligible could one refer either to the normal morphology of the 

 Pelmatozoa, or to the modes of life with which the assumed changes were connected. 

 But both of these courses would, at the present stage of the investigation, appear too 

 hypothetical. The morphology and physiology of the matter will be discussed later on. 

 Here, however, it must be pointed out that the compression cannot be described as 

 " dorsal- ventral," because in Pelmatozoa the dorsal pole of the theca is defined by the 

 attachment of the stem ; the dorso-ventral axis is normally a continuation of the 

 columnar axis ; and a dorso-ventral compression in the true sense would therefore be at 

 right angles to the compression of the Heterostelea. Similarly the terms " right " and 

 " left" must bear some relation to a sagittal plane ; but the sagittal plane of the Pelma- 

 tozoa is usually regarded as the plane passing through the dorsal pole, the ventral pole 

 where the mouth lies, and the hydropore (Bather, 1900, p. 20) ; and with whatever 

 the symmetry-plane of the Heterostelea may correspond, it certainly does not correspond 

 either with this or with any other plane known to the morphologist of Echinoderma, 

 so far as our knowledge enables us to judge. 



§ 23. In certain forms, and on certain hypotheses, the terms "right" and "left" 

 might be justified from a physiological standpoint and for purely practical convenience. 

 In Placocystis, for example, if we are justified in regarding the stem as posterior and 

 the other end of the theca as anterior, in the physiological aspect of locomotion ; and 



