760 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Asterids as having been longer and more closely connected than, in the 

 opinion of Dr. Semon, the facts warrant. It does not seem to be 

 justifiable to identify the Cystids with these indifferent stem-forms. We 

 know nothing of their internal organization, and what we do know of 

 their skeleton does not support this view. 



Embryology of Muelleria Agassizii.* — Mr. C. L. Edwards has 

 investigated the development of this common Holothurian. The eggs 

 are quite opaque, of a brownish colour, and surrounded by a struc- 

 tureless zona radiata. Three polar globules, one considerably larger 

 than the other two, are extruded; the segmentation is total and nearly 

 regular ; a gastrula is formed on the beginning of the second day. The 

 embryo becomes ciliated, and during the next day appears to pass through 

 an abbreviated auricularia-stage. F>y the fifth day the embryo developes 

 five oral tentacles and the beginning of the calcareous skeleton of the 

 larva, while green pigment-spots also aj)j)ear. On the fifth or sixth day 

 the embryo, by means of its tentacles, breaks the tough investing coat 

 and begins to creep about. On the sixth day an ambulacral foot arises 

 at the posterior end and grows so rapidly that, on the eighth day, it 

 exceeds the oral tentacles in length. On the eleventh day a second, and 

 on the fourteenth a thiixl ambulacral foot appears, and in such positions 

 as to diflerentiate the ventral sui'face. 



In the meantime the calcareous rods have been getting longer and 

 branching; the branches anastomose to make rosette form8, while from 

 the centre of each of these rosettes arise two vertical spines, which are at 

 first free and are then joined to one another by cross-bars. The intestine 

 is plainly visible as an orange-red body, and the anus is guarded by two 

 valves, formed of fused calcareous rods, running longitudinally to the 

 body, which wave, fan-like, from side to side with the cloacal respiratory 

 movements. Quite early a circulation of granules and corpuscles may 

 be noted in the ambulacral system. 



By the thirtieth day a fourth ventral foot has become developed, and 

 the budding fifth aj)pears, while, near to the base of the oral tentacles, 

 dorsally and laterally, there apj)ear respectively the beginnings of a pair 

 of ambulacral feet. By the forty-second day a fifth ventral foot has 

 partially developed, and two additional jDairs of lateral feet have 

 budded from the middle and posterior portitm of the sides. 



In the first day after hatching the sucking discs of the oral tentacles 

 have shown the beginning of division into two lobes ; somewhat later 

 these each divide into two lobes, giving the basis for the dichotomous 

 division of the tentacles in the adult. 



These Holothurians are not difficult to rear, and some were under 

 observation fur eighty-four days. 



Boring Sea-Urehiiis.| — Ilerr G. John has an essay on the somewhat 

 vexed question of the manner in which sea-urchins bore into rocks. He 

 comes to the conclusion that the cavities which they inhabit have been 

 formed by themselves, and that they produce them by means of their 

 masticatory apparatus, which is aided to some extent by their spines, as 

 the creatiu'es move round and round. These cavities are formed as a 

 protection against the action of the sea. I'he calcareous algfe which 

 cover the rocks inhabited by the sea-urchins are deposited mechanically, 



* Circ. John Hopkins Univ., viii. (1SS9) p. 37. 



t Arch. f. Naturgesch., Iv. (1889J pp. 268-302 (1 pi.). 



