6 H. F. NIERSTEASZ. 



this envelope is of chitinous character (Fig. 24). The tjvo extremities of the envelope 

 are tube-shaped (Fig. 24, B) ; it is open in the middle portion (Fig. 24, A), which may 

 have been caused by damage. Around the proper spicule a cellular envelope is found, 

 consisting of transparent epithelial cells with round nuclei, between which there are 

 long supporting cells with oblong nuclei (Fig. 24). Between the cellular envelope and 

 the spicule an almost homogeneous mass occurs, staining, pale-red with carmalum, which 

 I believe to be a secretion of the cells of the envelope, though perfect certainty cannot 

 be obtained. In this case the envelope might perform the duty of a poison gland, the 

 spicule having consequently the power of attack and defence. At the proximal 

 end of the spicules numerous small muscles are attached, which enable the animal 

 to move the spicules in various directions. 



The nervous system does not furnish much worthy of note. The cerebral 

 ganglion is large (Fig. 6, eg) ; the latero-ventral ganglia are large also (Fig. 7 , gr), 

 whereas the later o-dorsal ones are very small (Fig. 6, gd). As in other Proneomeniidce, 

 the cerebro-ventral and the cerebro-lateral connectives take their origin from the 

 cerebral ganglion separately. The buccal ganglia are distinct, as well as the buccal 

 commissure (sub-lingual commissure of Simroth), which takes its course dorsally from 

 the radula-sac (Fig. 10, he). The posterior superior ganglia are large (Fig. 21, gps), 

 the commissure between them strong. The posterior inferior ganglia are much smaller ; 

 the connectives between the posterior superior and inferior ganglia are strong. From 

 the posterior superior ganglia some strong nerves t§bke their course distally toward the 

 lip, situated behind the cloaca (Fig. 19, n). 



The heart shows some very interesting peculiarities. As far as can be observed, 

 the structure generally corresponds with that of the other forms, of which I have 

 already given a detailed account (8). Fig. 21 represents the proximal offsets of the 

 cloaca, which gradually disappear (cl). Ventrally from these the distal offsets of the 

 pericardium are found (/>), into one of which — the left — the left cloacal duct opens {cd). 

 The medial walls of these pericardial offsets invaginate ; these invaginations together 

 form the atria. There exists also a median distal offset of the pericardium (Fig. 21, p). 

 By the union of these three offsets the spacious pericardial cavity is formed ; at the 

 dorsal wall the two atrial invaginations still exist, separated (Fig. 22, a) ; more 

 proximally, however, they unite and form together the atrium. The double character 

 of the atrium remains here plainly visible. The atrium is consequently open on the 

 distal side ; on the dorsal side it is more or less closed by connective tissue. The 

 ventricle originates similarly to the atrium by the coalescence of two invaginations, 

 viz., the median walls of the pericardial ducts. The unpaired portion of the ventricle 

 unites with the dorsally situated atrium (Fig. 23, a, v). Concerning this we must 

 notice the following differences : in other forms the ventricle is situated dorsally from 

 the atrium ; it also arises from the union of two parts, but is continued as a dorsal 

 unpaired invagination at the dorsal pericardial wall, and communicates at its ventral 

 side with the atrium (8). In this form, however, the ventricle remains small and is not 



