ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 539 



usual conditions. When sixty hours old the larva becomes club-shaped, 

 the posterior tliickened region containing all the organs, and the anterior 

 region being apparently the homologue of the larval organ of Asterina 

 gibbosa and of the stalk of Antedon. As in Antedon the young larva is 

 uniformly ciliated, while the later larva has bands of cilia, and there are 

 some other resemblances between the larvae of the two forms. The 

 Ophiura larva shows no trace of the ordinary pluteus stage, but it is 

 nevertheless free-swimming and not developed in a brood-pouch. This 

 fact and the resemblances to the larva of Antedon lead the author to 

 believe that the larva is one of primitive type. The relation of the 

 larval symmetry to the adult is such that the ventral and dorsal regions 

 of the larva are equivalent to the oral and aboral regions of the adult, 

 there being no secondary twisting during development. 



Development of Cribrella oculata.* — Dr. A. T. Masterman pub- 

 lishes a preliminary note on this subject. He finds that segmentation 

 is at first total and unequal, but later free-cell formation occurs, and a 

 solid morula results whose cells are all of the same size. In some cases 

 normal gastrulation occurs, in others the hypoblast is formed by multi- 

 polar gastrulation, or by multipolar ingression of single cells. The 

 larva when hatched is uniformly ciliated, and shows an epiblast with a 

 pit at the posterior end which probably represents the blastopore. The 

 mesoblast is formed from two rudiments, one arising from the anterior 

 end of the hypoblast and the other from its posterior end. The anterior 

 forms right and left hydroccele and preoral ccelom, the posterior left 

 and right enteroccele. Of these, the preoral ccelom forms chiefly the 

 axial sinus, while the right hydroccele disappears ; the left enteroccele 

 gives rise mainly to the hypogastric ccelom and the right to the epi- 

 gastric ccelom. The anterior ccelomic rudiment gives rise to a small 

 thin-walled vesicle which resembles the preoral sac of Balanoglossus. 



Optic Organs of Starfishes.f — W. Pfeffer has made a study of these 

 organs which have been repeatedly described. The radial nerve ex- 

 pands at the tip of the arm, and thus forms a bed for the optic organ 

 which lies at the base of the " feeler," the blind end of the water- 

 vessel, — both being protected, when the ambulacral groove contracts, by 

 the spines and by the conically pointed terminal tube-feet without 

 suctorial discs. The optic cushion shows the same elements as are 

 found associated with the radial nerve itself, but in different proportions, 

 — e.g. supporting cells, cuticula-cells (forming cuticle and lens), and 

 epithelial sensory cells. The most important elements, however, are 

 tne retinal cells, each of w T hich consists of a cell-body, a thread-like 

 proximal portion, and, situated on the cell-body, the so-called rod, very 

 delicate and refractive. 



(1) In one group of species there is a diffuse distribution of the 

 retinal or optic cells over the whole optic cushion (Astropecten penta- 

 canthus and A. Mulleri) ; (2) in a second group, the optic cells are 

 restricted to certain regions, the optic grooves — e.g. Astropecten auran- 

 tiacus, Palmipes membranaceus, and Solaster papposus, with Luidia 

 ciliaris as intermediate between (1) and (2) ; (3) in a third group there 



* Proc. Roy. Phys. Soc. Edin., xiv. (1901) pp. 310-3 (1 pi.), 

 t Zool. Jahrb., xiv. (1901) pp. 523-50 (1 pi.). 



