XX THE VOYAGE OF H.M.S CHALLENGER. 



meridian planes, which are defined by the position of the four perradii and the four inter- 

 radii, and which stand in definite relation to the four parameres or pairs of antimeres 

 (comp. woodcut, fig. A). The four perradii lie in the two primary meridian planes (or 

 the " radial planes of Order I.") whilst the four interradii lie in the two secondary 

 mcridan planes (or the " radial planes of Order II."). Each of the two primary meridian 

 planes is therefore the middle plane (or saggital plane) of each two opposite parameres and 

 at the same time the boundary planes between the two symmetrical antimeres of each 

 paramere. On the other hand, each of the two secondary meridian planes is the boundary 

 plane (or septal plane) of the two adjacent parameres, and at the same time the boundary 

 plane of their two contiguous antimeres. If the four corners of the quadrate pyramid corre- 

 spond to the four perradii, its diagonal planes (in which each two opposite corners lie) are 

 the primary meridian planes, whilst the secondary meridian planes are formed by the 

 middle lines of each two opposite lateral surfaces of the pyramid. 



§ 30. Regular and amphitect pyramids. Whilst all Acraspedae and the majority of 

 Craspedotae retain the original primary fundamental form of the regular pyramid, in a 

 large number of Craspedotae (but in no Acraspedae) it passes into the secondary funda- 

 mental form of the bisected or amphitect pyramid. In the former, all the four parameres 

 of the body are completely congruent, whilst in the latter they are only congruent in 

 pairs, as each two adjacent parameres are symmetrically similar. Whilst the base of 

 the regularly quadrangular pyramid is a quadrate, that of the amphitect quadrangular 

 pyramid represents a rhombus. As in the amphitect pyramid the two primary transverse 

 axes are unequal in size, so, of the four parameres, each two adjacent are symmetrically 

 similar, each two opposite are congruent. In these amphitect Craspedotae it is usually 

 the unequal development of the tentacles which first causes the alteration of the regular 

 fundamental form. Of the four primary tentacles, the two opposite are much larger or the 

 only ones developed, whilst the other two, alternating with them, are smaller or quite 

 rudimentary (e.g., Thamnostylus, PL I. ; Discodonium, System, taf. i. fig. 6 ; Ctenaria, 

 System, taf. vii. figs. 3, 5 ; Dissonema, System, taf. viii. fig. 3 ; Dipetams, System, 

 taf. xviii. fig. 2 ; JEginella, System, taf. xx. fig. 16). The promorphological con- 

 ditions of the rhomboid pyramid are entirely the same in these amphitect four 

 Craspedotae as in the Ctenoplwra. In a few rare cases, the hexagonal amphitect pyramid 

 also appears along with the quadrangular, the same promorphism which distinguishes 

 most corals (e.g., Dipleurosoma, System, taf. ix. fig. 9). 



§31. Dipleuric or zeugite pyramids. The "pair pyramid," or zeugite pyramid, 

 appears much more rarely than the amphitect form, along with the predominating 

 regular pyramid as the geometrical fundamental form of the Medusae. In such cases the 

 whole Medusa persona has the same promorphological conditions as each one of the four 

 parameres in the ordinary regular Medusa persona. This condition is only found strongly 

 expressed in the Anthomedusa family of the Codonidae, and characterises there the 



