234 SUMMARY OF CURRENT RESEARCHES RELATING TO 



archenteron, multiply and send into the yolk nuclei, from wliicli new 

 endoderm cells are formed. 



The archenteron is obscured, and is never included within the yolk. 

 There is a great accumulation of nuclei at the side of the blastopore, just 

 below the blastoderm, and these become mesoderm cells, which also appear 

 in the abdominal region later on. Thus, the mesoderm arises partly from 

 the superficial and partly from the invaginated ectoderm. 



After the appearance of the nauplius stage, the development passes on 

 much in the same way as in the lobster. The development of the eye has 

 been traced through all the later stages in Alpheus and Palsemonites. 



f At the nauplius stage the brain and optic ganglia form a continuous 

 mass of ectoderm cells arising by proliferation of the superficial ectoderm. 

 There is no invaginated cavity such as Kingsley has described. The 

 optic ganglia nearly meet above the brain ; the superficial ectoderm 

 cells elongate, and from these cells alone the eye is formed. These divide 

 transversely, so as to form a series of radial strings. Separating these 

 from the underlying yolk, or, in some cases mesoderm, is a basal membrane 

 which soon becomes pigmented. The pigment cells elongate radially 

 outwards, and become the retinulfe. and each ommatidium of Alpheus, Peneeus, 

 and the other decapods examined, possesses seven retinulas. 



The outermost cells of the strings separate slightly from the inner 

 ones, and form the corneal hypodermis which secretes the cornea. Below 

 these follows a stratum of more elongated cells, the retinophoree ; these are 

 in groups of four, with white granular matter between each ; this is a 

 eecretion product, which will form the crystalline cones. The space 

 between the cornea and retinophoraa is filled by undifferentiated ectoderm 

 cells. Between and around the retinulas, a chitinous framework becomes 

 developed, which is continued below the basal membrane. 



The primitive ommatidium does not resemble an ocellus ; nor does the 

 development of the compound eye favour the supposition that it has arisen 

 by a gradual fusion of ocelli. 



Sense of Touch in Astacus.* — Mr. G. L. Gulland describes his experi- 

 ments and the results derived therefrom, as to the structure and distri- 

 bution of the setaB on Astacus. These setae are either sensory, in which case 

 the lumen communicates with the canal through the integument at the end 

 of which the seta is articulated ; or they are simply fringing setse, when 

 this lumen is closed, and no nerve can be traced into it. Of the sensory 

 setae — auditory, olfactory, or tactile — he discusses only the last, which are 

 most conveniently seen in the abdominal swimmerets. They are long, 

 simple, cylindrical at the base and hollow, with granules in the lumen — 

 the remains of a " papilla " of the hypodermis, which assist, after a moult, 

 in the formation of a new seta. In the fringing setae, the lumen is closed 

 near the proximal end by a chitinous ingrowth but in Thysanopoda this 

 closure is absent. A detailed account is given of the distribution of the 

 tactile setaB on the appendages and body oi Astacus. 



The nerve-endings were studied in the great chela. They are nearly 

 cylindrical, surrounded by a membrane, in direct continuity with the sur- 

 rounding connective tissue. Within the membrane is granular protoplasm, 

 in which are a number of nuclei resembling those of ganglion-cells. At 

 the proximal end of this tactile organ the nerve-fibres break up and become 

 continuous with its protoplasm. 1 here are generally two or three nerve-end 

 organs to each tuft of setse. Eacli nerve-fibre, after leaving the end-organ, 

 passes through the hypodermis, and breaks up, sending a branch into each 



* Proc. R. Phys. Soc. Edin., cxv. (1885-6) pp. 151-79 (2 pis.)- 



