56 ANATOMY OF THE ENOPLA. 



posterior, or slightly oblique ; but in the prepared specimen this is often transverse (Plate XI, 

 fig. 5). Each sac is ovoid in outline, has a thin, transparent, contractile investment (sufficiently 

 tough to prevent the points of the stylets piercing it during the motions of the worm), which lies 

 directly under the superficial layers of the division, and a duct passing from its central region 

 to communicate with the pit of the anterior chamber of the proboscis. The direction of the duct 

 in the position above mentioned (i. e., viewed as a transparent object) is forward and inward, 

 but, like other structures pertaining to this variable organ, it is liable to many alterations, and is 

 occasionally much stretched and attenuated. It is also slightly narrowed on approaching the 

 sac (Plate XIII, fig. 11, a), and has at its junction therewith a series of protecting fibres. De 

 Quatrefages and Max Schultze do not notice the duct at all, and Claparede's figure shows it 

 distorted from pressure in Tetrastemma, but Keferstein's representation is more accurate. Each 

 sac contains a variable number of the characteristic nail-shaped stylets (/3), from, three to five — 

 more or less — in different stages of development, as well as certain clear fluid vesicles (€), 

 globules and granules, and is quite filled by a transparent liquid. The stylets very much 

 resemble a lath-nail of cast-iron, and are formed of a translucent calcareous secretion ; indeed 

 they appear like spikes of the purest crystal. The head is bulged, rounded at the edges, and 

 somewhat flattened at the top, an elongated conical spike with a sharp point proceeding there- 

 from. The perfect spike or spikes in these sacs are usually about the size of the central stylet, 

 and there are often three or four that can scarcely be distinguished from each other. Besides 

 the perfect organs, there are some with heads not fully developed, but complete in other respects ; 

 the remainder again present the form of simple spikes of various lengths devoid of any head. In 

 a few instances the centre or axis both of the head and point of the stylet is granular, while the 

 superficial portion is of the usual homogeneous aspect. They seem to be formed by gradual increase 

 of layer upon layer of the calcareous glassy secretion, as is well shown in some specimens mounted 

 in chloride of calcium, which have assumed a stratified, or laminated appearance. Sometimes a 

 process (Plate X, fig. 18), probably the remains of a globule, passes from the head a short dis- 

 tance over the base of the spike, as indicated by Max Schultze in Tetrastemma, though seldom to 

 such an extent in the complete stylet. The knob on the head of the stylet figured by this author 

 must be rare, and probably represents a casual globule. The stylets are dissolved in weak 

 acetic acid, as first noted by M. de Quatrefages, and are roughened or corroded by a strong 

 solution of caustic potash. 



In a large animal (A. lactifioreus) an interesting arrangement of the stylet-sacs occurred on 

 one side, for there were two of nearly equal size (Plate XII, fig. 1), connected with each other at 

 one end, so that an interchange of fluid and granular contents took place. Only one had a duct of 

 communication with the anterior chamber of the proboscis. The opposite side was furnished with 

 a single sac of the usual formation, containing two large and perfect stylets, and a shorter without 

 a head. On the abnormal side the outer sac (in this view) had two well-formed stylets, a larger and 

 a smaller clear globule, besides some other minute globules and granules ; the inner, which 

 possessed the duct of communication, had one stylet as large as the preceding, and fully formed ; 

 another somewhat less, but also having a head ; a third slender spike of greater length than the 

 latter, but headless ; and a fourth, rather more than half the length of the last-mentioned. No 

 globule existed in the inner sac. It is interesting to notice the different degrees of perfection of 

 these spikes in relation to what Dr. Max Schultze avers as to their development, viz., that they are 

 the products of the smaller contained vesicles. In the one there were two large globules, and two 



