GEODIA MEDIA. 



201 



occur in both forms but are numerous in neither, have more or less curved rhabd- 

 omes 1.7-2.5 mm. long and 4-10 // thick at the cladomal end. The epirhabd 

 is 40-75 p. long, while the clade attains a length of 17-50 /«. The latter encloses 

 an angle of 32 89° with the epirhabd. Those in which this angle is small appear 

 as mesopromonaenes, those in which it is large as mesorthomonaenes, while the 

 intermediate forms are mesoplagiomonaenes. The clades are generally rounded 

 off terminally, more rarely pointed. The axial thread of the rhabdome and 

 epirhabd bears at the point of origin of the clade, besides the axial thread of the 

 clade itself, a number of small, rudimentary branchlets, resembling the struc- 

 tures observed by me in the anaclades of Thenea valdiviae} The axial thread 

 of the clade is rounded off at the end and often shows indications of being split 

 up terminally into small branches. Neither Bowerbank nor Sollas mentions 

 these spicules. I found several, however, in the former's type similar in every 

 respect, to those described above. 



I have found a few long and slender amphioxes of dimensions similar to 

 those of the rhabdomes (and epirhabds) of the mcsomonacnes. In some of 

 these spicules (Plate 17, fig. 7) a slight thickening near one end, enclosing a few 

 rudimentary branchlets of the axial thread, indicates clearly that they are 

 mesoclade-derivates, while in a few others hardly any trace of such a thickening, 

 or no thickening at all, can be detected. Although these spicules appear as 

 true amphioxes I am inclined to consider them as mesoclade-derivates. Such 

 spicules are not mentioned either by Bowerbank or by Sollas. I found several^ 

 however, in the former's type. 



The scarce anatriaenes have a long rhabdome, at the cladome about 12 fi 

 thick, and clades about 23 /« long and curved concave towards the rhabdome 

 throughout their whole length. The clade-angle is about 44°. Bowerbank 

 says that there are "very slight indications of the presence of recur vo-spicula," 

 Sollas, however, does not mention anatriaenes. In the spicule-preparations of 

 Bowerbank 's type, I observed a good many anatriaenes with rhabdomes 

 10-15 fj. thick, clades 25-35 /j. long, and clade-angles of 50-52°. I found in it 

 also a mesanatriacne with similar clades, an epirhabd 165 /( long, and a short- 

 ened, terminally thickened, club-shaped rhabdome, 0.8 mm. long and, at the 

 thickened, acladomal end, 27 fi thick. 



The large oxyasters and strongylasters (Plate 17, figs. 4-6b, 15b, 18b, 19b, 20) 

 are destitute of a central thickening and have from four to eleven, most fre- 



' R. V. Lcwienfdd. Die Tetiaxonia. Wissensch. ergebn. dcutschen Tiefsee-Expeflition, 1S98-1S99, 

 1907, 11, p. 200 ff. 



