NEPHEIDIA AND BODY-OAVITY OF DECAPOD CRUSTACEA. 409 



concretion in the distal portion of the tube. On the other 

 hand, in young specimens which have attained adult charac- 

 ters no trace of this gland has been detected. In them the 

 green gland, which did not appear to be functional at the time 

 of hatching, has become remarkably developed, its activity 

 having increased as that of the shell-gland diminished. 



Glaus has shown that shell-gland and green gland are deve- 

 loped in the same order in Nebalia as that which occurs in 

 Decapods, and the union of that animal with the Thoracos- 

 traca rather than with the Entomostraca is favoured (cf. 

 Glaus, No. 8, p. 100). 



The different behaviour of the two pairs of nephridia in 

 Entomostraca and Malacostraca renders it unlikely that the 

 latter have descended directly from the former. We should 

 rather suppose that the two groups possessed a common an- 

 cestor, in which both pairs of nephridia were equally deve- 

 loped. 



II. The Body-cavity. 



The Anterior Region of the Thorax. — Eig. 6 represents 

 a transverse section through the region of the second maxillee 

 of a Palsemonetes larva which is ten days old. The body 

 of the animal is surrounded by a layer of thin chitin, beneath 

 which the ectoderm lies. The latter consists of a layer of 

 flattened cells with oval nuclei. A considerable thickening of 

 the ectoderm is generally found in the middle dorsal line 

 (figs. 9, 16), whilst along the ventral surface the layer pre- 

 sents a peculiar granular appearance (figs. 6, 18, 19, ect.) 

 which I have been unable to account for. The cavity en- 

 closed by the chitin and ectoderm may be divided into four 

 regions : a dorsal sac (c.) surrounded by a definite layer of 

 epithelium, and in which the cephalic aorta (Glaus; "artere 

 ophthalmique" — Milne Edwards) lies (Ao. c), but which 

 does not itself contain blood; a central cavity {b, c), in 

 which the liver, intestine, and nerve-cord are found ; two 

 lateral cavities (6. lat.), separated from the central cavity 

 by masses of muscle and bands of connective tissue, and which 



VOL. XXXIV, PART IV. NEV7 SEE. E E 



