Nov. 27, 1 8 73 J 



NATURE 



73 



ON SOME RECENT RESULTS WITH THE 



TOWING NET ON THE SOUTH COAST OF 



IRELAND * 



I. — Mitraria 

 /~\NLY a single specimen was obtained of the little Mitraria, 

 ^-^ which formed the subject of the present communication, 

 and neither its structure nor devefopment was made out as com- 

 pletely as could have been wished. From the Mediterranean 

 species described in a former communication (British Association 

 Report for 1872), it differs in some points of structure and in the 

 mode of annulation of the developinij worm. It possesses the 

 usual Mitraria form, that of a hemispherical dome having its 

 base encircled by a band of long vibratile cilia. In the side of 

 the dome a little above the ciliated band is the mouth which 

 leads into a rather wide pharynx clothed with a ciliated epithelium. 

 The pharynx runs through tlie dome parallel to its base and opens 

 into a capacious stomach which continues in the same direction 

 until it joins the intestine. This then turns down abruptly at 

 right angles to tlie previous portion of the alimentary canal, and 

 then projects for a slight distance beyond the base of the dome, 

 carrying with it hernia-like the walls of the base. 



The true body walls of the future worm, of which i\ie Mitraria 

 is the larva, seem as yet confined to the intestinal segment of the 

 alimentary canal. They already present the commencement 

 of annulation, which, however, exists only on the dorsal and 

 ventral sides, while two broad bands of very distinct fibres may 

 be seen, one on the right and the other on the left side, extend- 

 ing transversely from the dorsal to the ventral surface. 



The ciliated band which runs round the base of the dome pos- 

 sesses a rather complex structure. It consists of two concentric 

 rings : an outer one composed of large oval distinctly nucleated 

 cells, and an inner one of a granular structure and yellowish 

 colour, in which no distinct cells could be demonstrated. The 

 cilia form two concentric wreaths borne by the under side of the 

 band, an outer wreatli consisting of very long cilia, and borne 

 by the inner edge of the outer portion of tlie band, and an 

 inner wreath of much shorter cilia borne by the inner 

 edge of the inner portion. The band with its cilia is 

 interrupted for a very short space at the aboral side of the 

 dome. There is probably at this spot an entrance into a water- 

 vascular system. No sucli system, however, was observed in the 

 specimen, though the author had described in another species of 

 Mitraria a system of sinuses which appear to exist in the walls 

 of the dome, and which he regarded as representing-,? water- 

 vascular system (Brit. Assoc. Report for 1872). 



Occupying the very summit of the dome is a large, somewhat 

 quadrilateral ganglion, from which two distinct filaments are 

 sent down, one on each side of the alimentary canal, but he was 

 not able to follow these filaments to their destination. The 

 bilateral symmetry of the ganglion suggests its formation out of 

 two lateral halves. Though its very superficial position gives it 

 the appearance of being a mere thickening of the walls, the view 

 here taken of its being a nervous ganglion seems to be the only 

 one consistent with its relations to the surrounding parts. 



On each side of the pharynx, a little behind the mouth, is a 

 small oval ganglion-like body from which a filament runs to the 

 ciliated band. Some delicate filaments may also be seen lying 

 between the pharynx and the walls of the dome on which they 

 seem to be distributed, but the author could not trace them to 

 any distinct ganglionic centre. 



The great apical ganglion carries two very obvious black ocel- 

 liform spots, and besides these two clear vesicles enclosing each 

 a clear spherical corpuscle. The two vesicles may probably be 

 regarded as auditory capsules. 



The further development of this larval form has not been ob- 

 served. It probably consists chiefly in the continued prolongation 

 of the alimentary canal beyond the base of the hemispherical 

 dome, the completion of the annulation by its extension to the 

 right and left sides, and tlie gradual contraction of the dome 

 and final absorption of the ciliated band. 

 2. — Tornaria 



Two specimens of the larval form originally discovered by 

 Johann Miiller, and described by him under the riame of lorna- 

 ria, were obtained, but these unfortunately perished before a 

 sufficiently exhaustive examination of tliem could be made. On 

 the whole their structure agrees closely with what has been 



• Paper read at the Meeting of the British Association, Bradford by 

 Prof. Alhnan, F.R.S. vv?! 



pointed out by Alex. Agassiz in his valuable and elaborate 

 memoir on Tornaria and Balano^lossns. The species appears 

 to be different from those hitherto described. The gills had not 

 begun to show themselves, and there were but traces of the 

 " lappets " described in other species as appended to the poste- 

 rior extremity of the stomach. 



The author believed that he could distinguish a minute gang- 

 lion on each side of the oesophagus ; filaments were sent off from 

 it to the neighbouring parts, and the two were connected to one 

 another by a sub-cesophagal commisure. The water-vascular 

 chamber was very distinct, but the so-called heart was not ob- 

 served ; while within the body-cavity, 1) ing close to the dorsa 

 pore and over the canal by which the great water-sac commu- 

 nicates with the external medium, was a small, closed, rather 

 thick-walled vesicle containing numerous oval corpuscles. Of 

 the nature of this vesicle the author could not offer any opinion. 



The cushion-like body which occupies the summit of the larva 

 exactly as in Mitraria, and supports the two ocelliform spots, 

 was very distinct, and so also was the contractile chord which 

 extends from this to the walls of the water-sac. The author, 

 however, could not here, any more than in Mitraria, regard the 

 cushion-like body as a mere thickening of the walls; he believed 

 it to be a nerve-mass, and thought he could trace two fine fila- 

 ments proceeding from it and running down, one towards the 

 right and the other towards the left side of the alimentary canal, 

 but he was not able to follow them for any distance, and he does 

 not regard their existence as confirmed. The extremely super- 

 ficial situation of this body, which makes it resemble a mere 

 thickness of the walls, is paralleled by that of the great ventral 

 nerve-mass in Sagitta. 



The contractile chord which runs to the water-sac is probably 

 attached to a capsular covering of the ganglion, rather than 

 directly to the ganglion itself. This chord, though showing 

 strong contractions by which the summit of the larva is drawn 

 down towards the water-sac, is of a homogeneous structure, pre- 

 senting no appearance of distinct fibrilla; or of other contractile 

 elements. 



The author instituted a comparison between Tornaria and 

 Mitraria. We have in both the external transparent pyramidal 

 or dome-shaped body, with a lateral oral orifice, and a basal 

 anal orifice, enclosing an alimentary canal which is divisible into 

 three regions, and takes a partly horizontal and partly vertical 

 direction in its course from one orifice to the other ;* we have in 

 both, near the base of the body, the circular band which carries 

 long vibratile cilia accompanied by a row of pigment spots, and 

 in both the cushion-hke ganglion carrying ocelli. 



From Mitraria, Tornaria chiefly difiers in the presence of the 

 thick sinuous and convoluted bands which give it so close a re- 

 semblance to certain Echinoderm larvae, and which are entirely 

 absent from Mitraria, and in its water- vascular system with the 

 contractile cord which extends from this to the apical ganglion. 

 If a water-vascular system is present in Mitraria, it consists 

 there of a system of sinuses excavated in the walls of the dome, 

 but without any representative of the great central sac. In 

 Mitraria the great apical ganglion carries not only the two 

 ocelli, but also two capsules, probably auditory ; these capsules 

 do not exist in Tornaria, In Mitraria the two nerve chords 

 which the apical ganglion sends down one on each side of the 

 alimentary canal are very distinct ; in Tornaria, if they exist at all, 

 they are by no means obvious. Finally, the ciliary circlet is 

 simple in Tornaria, while in iMitraria it is double. 



According to Alexander Agassiz's account of the development 

 of Tornaria into Balanoglossns, the great transverse circlet of 

 cilia becomes, by the elongation of the body, gradually pushed 

 backwards, so as to form the anal ciliated ring of the young 

 worm ; in Mitraria the great ciliary circlet remains unchanged 

 in position, and is probably ultimately absorbed, the worm 

 during its development acquiring a new anal wreath of cilia. 



3. Ametrangia hemispherica (nov. gen. et spec. ) 

 Among the most abundant products of the towing-net was a 

 little hydroid medusa, remarkable for the want of symmetry in 

 the distribution of its gastro-vascular canals. It is of a hemi- 

 spherical form, with the base about hall-an-inch in diameter, 

 and provided with very numerous (more than 100) marginal ten- 

 tacles, which are veiy extensile, and m.ay at one time be seen 

 floating away to a length of three or four inches and at another 

 coiled into a close spiral against the margin of the umbrella. 



♦ In the species of Mitraria described by J. Miiller and by Metschnikcff, 

 both oral and anal orifices arc basal, and the alimentary canal presents a u- 

 shaped curvature 



