ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 985 



layers, the middle longitudinal, sometimes with regular thicken- 

 ings, the internal oblique or transverse, sometimes with distinct 

 bundles. 



3. Ihe bristle* of Bonellia minor and of the two other types are 

 described in detail. Prof. Rietsch shows in all three the perfect 

 accordance in the structure and musculature of the anterior bristles. 

 These are absent in Hamingia, and the posterior bristles are peculiar 

 to Echiurus Rietsch agrees with Speugel in regarding the bristles 

 as ectodcrmic formations arising each from a single cell. 



4. The digestive tube is described in its three divisions — buccal, 

 intermediate, and anal. There is a great similarity in the different 

 types. The differences depend on the absence in Thalassemia of a 

 fine layer of longitudinal fibres in the anterior region of the buccal 

 intestine, in the presence of a diverticulum or caecum, and in the 

 presence (iu Th. Neptuni) of a (glandular?) ampulla or dilatation at 

 the end of the anal intestine. The Echiurus of Pallas exhibits a 

 special glandular region at the anus. The characteristic peculiarities 

 of the intestine are throughout the same, the change in the disposi- 

 tion of the two muscular layers behind the crop, the intimate relations 

 between the anterior portion of the intermediate intestine and the 

 vascular system by means of a ring or sinus, which perhaps secures 

 the nutrition of the blood and thus of the proboscis, the presence of a 

 vibratile groove and of a collateral intestine, with more or less 

 differentiated musculature. An analogous collateral intestine occurs 

 in the Capitellidas and in the Echinodermata. It has been comj)ared 

 to a transitory cellular strand observed in some Vertebrate embryos 

 (Selaehia, Teleostei, Batrachia). Balfour describes its formation in 

 Selachia as due to a median dorsal thickening of the intestinal wall, 

 or to a hollowed groove containing a prolongation of the intestinal 

 lumen. 



5. Anal glands. — The glands situated at the anus are very charac- 

 teristic of Eehiurians. They are simple in Echiurus and Thalassemia, 

 doubly ramified in Hamingia and B. viridis, and simply ramified in 

 B. minor. They are above all excretory. The currents caused by 

 the cilia of the funnels, in the gland itself, and in the terminal 

 portion of the intestine, could only cause a current towards the 

 exterior. Water may however pass from the glands into the peri- 

 visceral fluid, especially in Echiurus and Thalassemia. Their respira- 

 tory function remains doubtful. They have been compared to the 

 organ of Bojanus, to segmental organs, and to diverticula of the 

 digestive tube. Rietsch does not, however, come to any definite con- 

 clusion as to their homology. 



6. Nervous system. — The nerve-trunk has in adult Eehiurians 

 lost all trace of segmentation. The bilateral symmetry is, however, 

 sharply accented, not only by the more or legs pronounced dorsal 

 groove, but especially by the disposition of the ganglionic cells in two 

 longitudinal bauds. Th. erythrogr amnion is an exception in having 

 its ganglion-cell disposed in a single ventral baud. As in certain 

 annelids, the medulla and the branches of the collar are protected by 

 a neural canal, absent in Bonellias. The nerves are symmetrical, 



