38 



LUCERNARI^] AND THEIR ALLIES. 



periphery. The apparently jointed structure of the pennate bands does not belong 

 to tliem, hut is tlic result of the contraction of the muscular layer, along these lines, 

 which wrinkles the opsophragnia, at stated points, transversely to the trend of the 

 bauds. A few experiments with the point of a needle will soon convince one that 

 tlu! irritability of the animal may be exhibited by contractions at single points, or 

 along certain lines, as well as by a general shrinking of the body. One of these 

 bands may thus be impelled to contract so strongly as to produce heavy folds in 

 tlie opsophragma along that line, while it remains quite smooth on each side of it. 

 Before we enter into a more intimate research upon the structure of this part 

 of the muscular system, we should make this superficial reconnoissance complete 

 by taking special notice of the marginal terminus of the opsomyoplax, although it 

 has already been characterized in a general way. This terminus has the form of a 

 finely plaited border (m'), the plaits running parallel to its sides, and fading out 

 near the groups of tentacles. Except at the last place mentioned,, it is well 

 marked in character at all points of the periphery, but particularly pronounced 

 {fig. 25, m') at the bases of the anchors; the latter category standing in strong 

 contrast with its smoothness in the neighborhood of the tentacles, the homologues 

 of the anchors. 



82. We will proceed now to revise, from another point of view, the several areas 

 which we have just passed over, commencing with those which are most directly 

 continuous with the parietcs of the proboscis. These we shall find between the 

 corners of the buttresses (i,"') and the umbellar margin. By bringing the muscular 

 layer here into profile view, by means of a transverse section {fig. 62, m), we 

 disclose the fundamental element which lies at the bottom of the striation, noticed 

 above. This turns out to be a more or less extended thickening of the stratum 

 along stated lines, which run in the direction indicated by the striae. These 

 thickenings appear in the form of narrow ridges of varying height in other areas, 

 but here they are quite low, and do not rise at very sharp angles from the general 

 mass, yet they are of sufficient altitude to produce, by their comparative opacity, 

 a distinct contrast with the transparency of the thinner intervals ; and hence arise 

 the lighter and darker lines, which have been spoken of as striae. Approaching 

 the margin of the umbella the striae grow stronger, and the ridges become corre- 

 spondingly higher, at first no greater in altitude than in breadth ; but finally, as 

 the marginal, plaited band is entered, they abruptly increase to the proportions of 

 very lofty narrow and thin crests (jrt'), with intervals of breadth equal to them, 

 between their bases. At the less elevated points the ridges do not encroach upon 

 the chondromyoplax (/>) very sensibly, but as they become more prominent they 

 plunge deeper into its anterior face, and within the marginal, plaited band they 

 cut nearly through it, and, in fact, occupy as much, if not more, of the anterior 

 parietes of the umbella along its border, than the layer in which they are imbedded, 

 and have partially displaced. At last the muscular stratum meets the abrupt, con- 

 vex margin of i\w. chondrophys (c), close to the under surface of the opsophragma. 

 This is not its termination, however ; that is to be found along the line at which 

 the inner layer (o*) of tlie chondrophys thins out to a sharp edge ; and in order to 

 reach this place the muscle makes a final plunge between the abrupt convex margin 



