ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 55 



indicates the margin of the chitinous covering. The eyes appear as 

 two red points ; cilia commence to move, at first on the infra-buccal 

 pit, then on the mouth, and finally on the top of the head, where they 

 form a circlet. The armature of the mastax comes into existence, the 

 tail retires little by little towards the extremity of the egg, whose 

 envelope it finally ruptures. The larva has been already described by 

 several authors, and M. Joliet only insists on the fact that, like the 

 larva of Lacinularia figured by Huxley, it presents cilia on three parts 

 of its body ; a continuous, and at this time scarcely sinuous circlet 

 placed above the mouth ; a second circlet surrounding this and the 

 mouth, stretching even over the vibratile pit ; lastly, a tuft of cilia at 

 the extremity of the tail. The larva remains active for some hours, 

 and then becomes fixed by means of the glands enclosed in its tail. It 

 then commences to collect in its vibratile pit the minute particles held 

 in suspension in the water, mixes them with the secretion from the 

 gland, hitherto taken for a ganglion, and, according to the judicious 

 observations of Gosse and Williamson, therewith forms the pellets 

 which, when juxtaposed, constitute the tube it inhabits. 



Ech.inodermata. 



Development of the Skeleton of the Ophiurida.* — The first 

 point to which Prof. H. Ludwig addresses himself is the develo]3ment 

 of the arm-ossicles ; these he has previously stated to be originally 

 double, but he has never till now been able to demonstrate this by a 

 reference to embryological data, though the discovery by Lyman of 

 deep-sea forms in which these ossicles were distinctly double has 

 afforded considerable support to Dr. Ludwig's doctrine. The form 

 best adapted for investigation is the viviparous Amphmra squamata. 



As is well known, the arms of the O^^hiurid grow at the tip ; the 

 first rudiment of the ossicle consists of two calcareous pieces symme- 

 trically placed on either side of the middle line of the arm, and each 

 has somewhat of a triangular form ; one ray is directed aborally in 

 the long axis of the arm, the other two look adorally, and form 

 between them a smaller angle than each of them forms with the aboral 

 piece ; these two do not, however, lie in the same plane, but one is 

 dorsal and the other ventral, the former being further median and the 

 other lateral in position. At an early period a distinct difterence may 

 be seen in the size of these three rays ; the aboral becomes longer 

 than the adoral rays ; the form of the whole piece changes, owing to the 

 development of calcareous processes, which sooner or later fork at their 

 free end, and become connected with the ends of neighbouring forks, 

 so as to give rise to the reticular tissue characteristic of the Echino- 

 dermata. In this way the two adoral pieces become connected together. 

 Soon, too, the aboral process begins to form meshworks. This mode 

 of growth not only takes place laterally but also mesially, so that 

 the ends of the adjoining ossicles come into direct contact, without, 

 however, fusing. Later on, this fusion commences both at the aboral 

 and adoral ends : in their middle there is a space with concave sides, 

 which only becomes completely filled uj) at a later stage. 



* Zeitsch. f. wiss. Zool., xxxvi. (1S81) pp. 181-209 (2 pis.). 



