510 DR F, A. BATHER. 



the majority are preserved with the antanal face uppermost, it does not necessarily 

 follow that this was the position during life. For, assume the contrary, then the dead 

 theca resting on the three umbones, and deprived of the balance given as required 

 during life by arms and stem, would easily be overturned by a slight disturbance. If, 

 on the other hand, the flat anal face had been downwards from the beginning, it would 

 not so easily have been overturned. 



§ 610. If Pleurocystis were a freely errant form, one would find it very difficult to 

 give up the idea that it rested and progressed on its anal face, just as some of the later 

 Anomalocystidae may have slid along on the down-bent edges of their antanal face. 

 But the well-developed stem of Pleurocystis, with its distal coil, implies habitual, if 

 not precisely permanent, attachment. Thus, somewhat to my surprise, reasoned 

 argument leads me to the following conception. 



§ 611. A Pleurocystis coiled the end of its stem loosely round some upright object, 

 and stretched out its theca parallel with the sea-floor, and with the anal face upper- 

 most. In this position it resembled Dendrocystis, but from that genus it probably 

 diff"ered in having the theca much closer to the bottom, and indeed resting on it as a 

 rule, or at least in contact with the loose sandy ooze. Thus, in the clearer waters of 

 the Trenton Limestone, the brachioles swept in their food-stream, the periproct dilated 

 as the stomach filled, and the pectinirhombs breathed freely the while. But, in the 

 sandier waters of the British shore-line, the pectinirhombs were clogged, and increased 

 work was thrown on the rectum. This in its turn brought about a widening of the 

 periproct and all the changes of structure that have been detailed in these pages. 



§ 612. It was my original intention to conclude this discussion with some general 

 observations on the assumption of an eleutherozoic habit by Pelmatozoa, and in 

 particular by Cystidea. But the recent publication of Dr Kirk's survey of the whole 

 field and of his well-argued conclusions (1911) removes the need for any such elaborate 

 treatment. There are just one or two points that may be emphasised. 



It is certainly remarkable that at so early a period the true Pelmatozoa should so 

 frequently have given up what to most people's minds is the main character of a 

 Pelmatozoon — permanent attachment by a stem. But the true criterion of a Pelma- 

 tozoon is the mode of feeding by a subvective system ; and while this is the natural 

 accompaniment of a statozoic existence, the formation of a stem may be no more than 

 an adaptation to special and localised conditions. If anyone talks of Pelmatozoa 

 becoming eleutherozoic, he certainly does not mean that they become Eleutherozoa. In 

 all essential morphological and physiological characters they remain Pelmatozoa. Rarely, 

 indeed, can it be said that Pelmatozoa assume an actively errant life, roaming about 

 seeking what they may devour. The suggestion has been made in the case of the 

 later Anomalocystidae, but, even if it hold good for them, there are few other Pelmatozoa 

 for which the supposition would be possible. 



But to keep to the forms discussed in this memoir, the conclusion reached is that 

 all, even the most highly modified, were essentially statozoic Pelmatozoa. Their 



