16 M. 
ECHINODERMATA. 
Hertwig describes the behaviour of the nucleus during the maturation 
of parthenogenetic ova of Asterias and Astropecten. After the first polar 
body is formed, a second nuclear spindle is formed beneath it, leading to a 
second division of the nucleus. Instead, however, of a second polar body 
being formed, the two halves of the nucleus travel to the centre of the 
ovum, and fuse to form a first segmentation nucleus, which soon com- 
mences to divide and the ovum to segment. 
Janet describes a specimen of E chinocorys carinatus , in which three of 
the four genital plates have two pores each, the fourth plate three. 
Ludwig (1) observes that Ophiocnida sexradia , Dune., can increase by 
transverse division like Ophiactis virena (Sars), Liitken. 
According to Sturtz, the five oral shields of recent Ophiuroids are 
always wanting in pahnozoic forms, in which respect the recent Aatro- 
phiura permira, Sladen, with only one oral shield present as the madre- 
porite, is perhaps intermediate between palaeozoic and recent forms. The 
oral angle shields, lateral oral shields, jaws, jaw-plates, and oral papillae are 
normally developed in palaeozoic forms, but the radial shields have never 
been observed in any of them. The arm skeleton of palaeozoic Ophiuroids 
shows various stages of development : — (1) In Ophioencrinasterice the hard 
parts were enveloped in a soft skin ; ventral and dorsal shields were 
wanting ; the vertebrae seen from below consist of unconnected alter- 
nating halves, and the ambulacral pores are either in the vertebrae or 
between these and the lateral shields : (2) in Ophiura lymani the 
dorsal shields are wanting ; the inner arm skeleton consists of rod-like 
bodies, but those of each pair are opposite to one another and fixed 
at the ends ; (3) forms such as Furcaster, &c., which resemble (4), but 
in which ventral shields are doubtfully present ; (4) palaeozoic Ophiu- 
rids similar to recent forms, but without dorsal shields ; the ambu- 
lacral vertebrae are normal. Radial and oral shields wanting. The 
author supports Ludwig in homologizing the vertebrae of Ophiurids, 
and not their ventral shields, with the ambulacrals of Stellerids, since 
in the oldest Ophiurids the halves of the vertebrae are disconnected 
and often alternating, while the ventral shields are often wanting. 
In several Encrinasterice , as well as in some palaeozoic Stelleridce 
verce , the madreporite is on the ventral surface, while in other Encri- 
nasterice it is dorsal, as in recent forms. The mouth structure of palae- 
ozoic starfish seems to be similar to that of recent forms. Encri- 
nasterice with alternating ambulacral plates are to be distinguished 
from Stelleridce with the ambulacral plates of each pair opposite to one 
another ; but, apart from this peculiarity, the Encrinasterice stand as close 
to recent Stelleridce as do the palaeozoic Stelleridce verce. 
The author looks upon the Cystidea as ancestors of the Asteroidea ) 
through such forms as Edrioaster , Agelacrinus, and Uemicystites. He con- 
siders the third opening of many Cystids to have been perhaps that of the 
stone canal. If an originally spherical or oval Cystid spreads out to a 
flat or disc-shaped form, with the three openings on the upper side, we get 
a transitional form like Agelacrinus. At the same time the external hard 
